Cite as: 567 U. S. ____ (2012)

1

Opinion of the Court
NOTICE: This opinion is subject to formal revision before publication in the
preliminary print of the United States Reports. Readers are requested to
notify the Reporter of Decisions, Supreme Court of the United States, Wash­
ington, D. C. 20543, of any typographical or other formal errors, in order
that corrections may be made before the preliminary print goes to press.

SUPREME COURT OF THE UNITED STATES
_________________

No. 11–182
_________________

ARIZONA, ET AL., PETITIONERS v. UNITED STATES
ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF


APPEALS FOR THE NINTH CIRCUIT


[June 25, 2012] 


JUSTICE KENNEDY delivered the opinion of the Court.
To address pressing issues related to the large number
of aliens within its borders who do not have a lawful right
to be in this country, the State of Arizona in 2010 enacted
a statute called the Support Our Law Enforcement and
Safe Neighborhoods Act. The law is often referred to as
S. B. 1070, the version introduced in the state senate. See
also H. 2162 (2010) (amending S. 1070). Its stated pur­
pose is to “discourage and deter the unlawful entry and
presence of aliens and economic activity by persons unlaw­
fully present in the United States.” Note following Ariz.
Rev. Stat. Ann. §11–1051 (West 2012). The law’s provi­
sions establish an official state policy of “attrition through
enforcement.” Ibid. The question before the Court is
whether federal law preempts and renders invalid four
separate provisions of the state law.
I
The United States filed this suit against Arizona, seek­
ing to enjoin S. B. 1070 as preempted. Four provisions of
the law are at issue here. Two create new state offenses.
Section 3 makes failure to comply with federal alien­
registration requirements a state misdemeanor. Ariz.

 2

ARIZONA v. UNITED STATES
Opinion of the Court

Rev. Stat. Ann. §13–1509 (West Supp. 2011). Section 5, in
relevant part, makes it a misdemeanor for an unauthor­
ized alien to seek or engage in work in the State; this
provision is referred to as §5(C). See §13–2928(C). Two
other provisions give specific arrest authority and investigative duties with respect to certain aliens to state and
local law enforcement officers. Section 6 authorizes offic­
ers to arrest without a warrant a person “the officer has
probable cause to believe . . . has committed any public
offense that makes the person removable from the United
States.” §13–3883(A)(5). Section 2(B) provides that offic­
ers who conduct a stop, detention, or arrest must in some
circumstances make efforts to verify the person’s immi­
gration status with the Federal Government. See §11–
1051(B) (West 2012).
The United States District Court for the District of
Arizona issued a preliminary injunction preventing the
four provisions at issue from taking effect. 703 F. Supp.
2d 980, 1008 (2010). The Court of Appeals for the Ninth
Circuit affirmed. 641 F. 3d 339, 366 (2011). It agreed that
the United States had established a likelihood of success
on its preemption claims. The Court of Appeals was unan­
imous in its conclusion that §§3 and 5(C) were likely
preempted. Judge Bea dissented from the decision to
uphold the preliminary injunction against §§2(B) and 6.
This Court granted certiorari to resolve important ques­
tions concerning the interaction of state and federal power
with respect to the law of immigration and alien status.
565 U. S. ___ (2011).
II


A

The Government of the United States has broad, un­
doubted power over the subject of immigration and the
status of aliens. See Toll v. Moreno, 458 U. S. 1, 10 (1982);
see generally S. Legomsky & C. Rodríguez, Immigration

 Cite as: 567 U. S. ____ (2012)

3

Opinion of the Court

and Refugee Law and Policy 115–132 (5th ed. 2009). This
authority rests, in part, on the National Government’s
constitutional power to “establish an uniform Rule of Naturalization,” U. S. Const., Art. I, §8, cl. 4, and its inherent power as sovereign to control and conduct relations
with foreign nations, see Toll, supra, at 10 (citing United
States v. Curtiss-Wright Export Corp., 299 U. S. 304, 318
(1936)).
The federal power to determine immigration policy is
well settled. Immigration policy can affect trade, invest­
ment, tourism, and diplomatic relations for the entire
Nation, as well as the perceptions and expectations of
aliens in this country who seek the full protection of its
laws. See, e.g., Brief for Argentina et al. as Amici Curiae;
see also Harisiades v. Shaughnessy, 342 U. S. 580,
588–589 (1952). Perceived mistreatment of aliens in the
United States may lead to harmful reciprocal treatment
of American citizens abroad. See Brief for Madeleine K.
Albright et al. as Amici Curiae 24–30.
It is fundamental that foreign countries concerned about
the status, safety, and security of their nationals in the
United States must be able to confer and communicate on
this subject with one national sovereign, not the 50 sepa­
rate States. See Chy Lung v. Freeman, 92 U. S. 275, 279–
280 (1876); see also The Federalist No. 3, p. 39 (C. Rossiter
ed. 2003) (J. Jay) (observing that federal power would be
necessary in part because “bordering States . . . under the
impulse of sudden irritation, and a quick sense of appar­
ent interest or injury” might take action that would un­
dermine foreign relations). This Court has reaffirmed that
“[o]ne of the most important and delicate of all interna­
tional relationships . . . has to do with the protection of the
just rights of a country’s own nationals when those na­
tionals are in another country.” Hines v. Davidowitz, 312
U. S. 52, 64 (1941).
Federal governance of immigration and alien status is

 4

ARIZONA v. UNITED STATES
Opinion of the Court

extensive and complex. Congress has specified catego­
ries of aliens who may not be admitted to the United
States. See 8 U. S. C. §1182. Unlawful entry and unlawful
reentry into the country are federal offenses. §§1325,
1326. Once here, aliens are required to register with the
Federal Government and to carry proof of status on their
person. See §§1301–1306. Failure to do so is a federal
misdemeanor. §§1304(e), 1306(a). Federal law also au­
thorizes States to deny noncitizens a range of public bene­
fits, §1622; and it imposes sanctions on employers who
hire unauthorized workers, §1324a.
Congress has specified which aliens may be removed
from the United States and the procedures for doing so.
Aliens may be removed if they were inadmissible at the
time of entry, have been convicted of certain crimes, or
meet other criteria set by federal law. See §1227. Re­
moval is a civil, not criminal, matter. A principal feature of
the removal system is the broad discretion exercised by
immigration officials. See Brief for Former Commission­
ers of the United States Immigration and Naturalization
Service as Amici Curiae 8–13 (hereinafter Brief for For­
mer INS Commissioners). Federal officials, as an initial
matter, must decide whether it makes sense to pursue
removal at all. If removal proceedings commence, aliens
may seek asylum and other discretionary relief allowing
them to remain in the country or at least to leave without
formal removal. See §1229a(c)(4); see also, e.g., §§1158
(asylum), 1229b (cancellation of removal), 1229c (volun­
tary departure).
Discretion in the enforcement of immigration law em­
braces immediate human concerns. Unauthorized work­
ers trying to support their families, for example, likely
pose less danger than alien smugglers or aliens who com­
mit a serious crime. The equities of an individual case
may turn on many factors, including whether the alien
has children born in the United States, long ties to the

 Cite as: 567 U. S. ____ (2012)

5

Opinion of the Court

community, or a record of distinguished military service.
Some discretionary decisions involve policy choices that
bear on this Nation’s international relations. Returning
an alien to his own country may be deemed inappropriate
even where he has committed a removable offense or fails
to meet the criteria for admission. The foreign state may
be mired in civil war, complicit in political persecution, or
enduring conditions that create a real risk that the alien
or his family will be harmed upon return. The dynamic
nature of relations with other countries requires the Exec­
utive Branch to ensure that enforcement policies are con­
sistent with this Nation’s foreign policy with respect to
these and other realities.
Agencies in the Department of Homeland Security play
a major role in enforcing the country’s immigration laws.
United States Customs and Border Protection (CBP) is responsible for determining the admissibility of aliens and
securing the country’s borders. See Dept. of Homeland
Security, Office of Immigration Statistics, Immigration
Enforcement Actions: 2010, p. 1 (2011). In 2010, CBP’s
Border Patrol apprehended almost half a million people.
Id., at 3. Immigration and Customs Enforcement (ICE), a
second agency, “conducts criminal investigations involving
the enforcement of immigration-related statutes.” Id., at
2. ICE also operates the Law Enforcement Support Cen­
ter. LESC, as the Center is known, provides immigration status information to federal, state, and local officials
around the clock. See App. 91. ICE officers are respon­
sible “for the identification, apprehension, and removal of
illegal aliens from the United States.” Immigration En­
forcement Actions, supra, at 2. Hundreds of thousands of
aliens are removed by the Federal Government every year.
See id., at 4 (reporting there were 387,242 removals, and
476,405 returns without a removal order, in 2010).

 6

ARIZONA v. UNITED STATES
Opinion of the Court

B
The pervasiveness of federal regulation does not di­
minish the importance of immigration policy to the States.
Arizona bears many of the consequences of unlawful im­
migration. Hundreds of thousands of deportable aliens
are apprehended in Arizona each year. Dept. of Homeland
Security, Office of Immigration Statistics, 2010 Yearbook
of Immigration Statistics 93 (2011) (Table 35). Unauthor­
ized aliens who remain in the State comprise, by one estimate, almost six percent of the population. See Passel
& Cohn, Pew Hispanic Center, U. S. Unauthorized Immigration Flows Are Down Sharply Since Mid-Decade 3
(2010). And in the State’s most populous county, these
aliens are reported to be responsible for a disproportionate
share of serious crime. See, e.g., Camarota & Vaughan,
Center for Immigration Studies, Immigration and Crime:
Assessing a Conflicted Situation 16 (2009) (Table 3) (esti­
mating that unauthorized aliens comprise 8.9% of the
population and are responsible for 21.8% of the felonies in
Maricopa County, which includes Phoenix).
Statistics alone do not capture the full extent of Arizo­
na’s concerns. Accounts in the record suggest there is an
“epidemic of crime, safety risks, serious property damage,
and environmental problems” associated with the influx
of illegal migration across private land near the Mexican
border. Brief for Petitioners 6. Phoenix is a major city of
the United States, yet signs along an interstate highway
30 miles to the south warn the public to stay away. One
reads, “DANGER—PUBLIC WARNING—TRAVEL NOT
RECOMMENDED / Active Drug and Human Smuggling
Area / Visitors May Encounter Armed Criminals and
Smuggling Vehicles Traveling at High Rates of Speed.”
App. 170; see also Brief for Petitioners 5–6. The problems
posed to the State by illegal immigration must not be
underestimated.
These concerns are the background for the formal legal

 Cite as: 567 U. S. ____ (2012)

7

Opinion of the Court

analysis that follows. The issue is whether, under pre­
emption principles, federal law permits Arizona to imple­
ment the state-law provisions in dispute.
III
Federalism, central to the constitutional design, adopts
the principle that both the National and State Govern­
ments have elements of sovereignty the other is bound to
respect. See Gregory v. Ashcroft, 501 U. S. 452, 457
(1991); U. S. Term Limits, Inc. v. Thornton, 514 U. S. 779,
838 (1995) (KENNEDY, J., concurring). From the existence
of two sovereigns follows the possibility that laws can be
in conflict or at cross-purposes. The Supremacy Clause
provides a clear rule that federal law “shall be the su­
preme Law of the Land; and the Judges in every State
shall be bound thereby, any Thing in the Constitution or
Laws of any State to the Contrary notwithstanding.”
Art. VI, cl. 2. Under this principle, Congress has the
power to preempt state law. See Crosby v. National Foreign Trade Council, 530 U. S. 363, 372 (2000); Gibbons
v. Ogden, 9 Wheat. 1, 210–211 (1824). There is no doubt
that Congress may withdraw specified powers from the
States by enacting a statute containing an express
preemption provision. See, e.g., Chamber of Commerce of
United States of America v. Whiting, 563 U. S. ___, ___
(2011) (slip op., at 4).
State law must also give way to federal law in at least
two other circumstances. First, the States are precluded
from regulating conduct in a field that Congress, acting
within its proper authority, has determined must be regu­
lated by its exclusive governance. See Gade v. National
Solid Wastes Management Assn., 505 U. S. 88, 115 (1992).
The intent to displace state law altogether can be inferred
from a framework of regulation “so pervasive . . . that
Congress left no room for the States to supplement it” or
where there is a “federal interest . . . so dominant that the

 8

ARIZONA v. UNITED STATES
Opinion of the Court

federal system will be assumed to preclude enforcement of
state laws on the same subject.” Rice v. Santa Fe Elevator
Corp., 331 U. S. 218, 230 (1947); see English v. General
Elec. Co., 496 U. S. 72, 79 (1990).
Second, state laws are preempted when they conflict
with federal law. Crosby, supra, at 372. This includes
cases where “compliance with both federal and state
regulations is a physical impossibility,” Florida Lime &
Avocado Growers, Inc. v. Paul, 373 U. S. 132, 142–143
(1963), and those instances where the challenged state law
“stands as an obstacle to the accomplishment and execu­
tion of the full purposes and objectives of Congress,”
Hines, 312 U. S., at 67; see also Crosby, supra, at 373
(“What is a sufficient obstacle is a matter of judgment, to
be informed by examining the federal statute as a whole
and identifying its purpose and intended effects”). In
preemption analysis, courts should assume that “the
historic police powers of the States” are not superseded
“unless that was the clear and manifest purpose of Con­
gress.” Rice, supra, at 230; see Wyeth v. Levine, 555 U. S.
555, 565 (2009).
The four challenged provisions of the state law each
must be examined under these preemption principles.
IV
A
Section 3
Section 3 of S. B. 1070 creates a new state misdemeanor. It forbids the “willful failure to complete or carry an
alien registration document . . . in violation of 8 United
States Code section 1304(e) or 1306(a).” Ariz. Rev. Stat.
Ann. §11–1509(A) (West Supp. 2011). In effect, §3 adds a
state-law penalty for conduct proscribed by federal law.
The United States contends that this state enforcement
mechanism intrudes on the field of alien registration, a
field in which Congress has left no room for States to

 Cite as: 567 U. S. ____ (2012)

9

Opinion of the Court

regulate. See Brief for United States 27, 31.
The Court discussed federal alien-registration require­
ments in Hines v. Davidowitz, 312 U. S. 52. In 1940, as
international conflict spread, Congress added to federal
immigration law a “complete system for alien registra­
tion.” Id., at 70. The new federal law struck a careful
balance. It punished an alien’s willful failure to register
but did not require aliens to carry identification cards.
There were also limits on the sharing of registration rec­
ords and fingerprints. The Court found that Congress
intended the federal plan for registration to be a “single
integrated and all-embracing system.” Id., at 74. Because
this “complete scheme . . . for the registration of aliens”
touched on foreign relations, it did not allow the States to
“curtail or complement” federal law or to “enforce addi­
tional or auxiliary regulations.” Id., at 66–67. As a con­
sequence, the Court ruled that Pennsylvania could not
enforce its own alien-registration program. See id., at 59, 74.
The present regime of federal regulation is not identi­
cal to the statutory framework considered in Hines, but
it remains comprehensive. Federal law now includes a
requirement that aliens carry proof of registration. 8
U. S. C. §1304(e). Other aspects, however, have stayed the
same. Aliens who remain in the country for more than 30
days must apply for registration and be fingerprinted.
Compare §1302(a) with id., §452(a) (1940 ed.). Detailed
information is required, and any change of address has
to be reported to the Federal Government. Compare
§§1304(a), 1305(a) (2006 ed.), with id., §§455(a), 456 (1940
ed.). The statute continues to provide penalties for the
willful failure to register. Compare §1306(a) (2006 ed.),
with id., §457 (1940 ed.).
The framework enacted by Congress leads to the conclu­
sion here, as it did in Hines, that the Federal Government
has occupied the field of alien registration. See American
Ins. Assn. v. Garamendi, 539 U. S. 396, 419, n. 11 (2003)

 10

ARIZONA v. UNITED STATES
Opinion of the Court

(characterizing Hines as a field preemption case); Pennsylvania v. Nelson, 350 U. S. 497, 504 (1956) (same); see also
Dinh, Reassessing the Law of Preemption, 88 Geo. L. J.
2085, 2098–2099, 2107 (2000) (same). The federal statu­
tory directives provide a full set of standards governing
alien registration, including the punishment for noncom­
pliance. It was designed as a “ ‘harmonious whole.’ ”
Hines, supra, at 72. Where Congress occupies an entire
field, as it has in the field of alien registration, even com­
plementary state regulation is impermissible. Field pre­
emption reflects a congressional decision to foreclose any
state regulation in the area, even if it is parallel to federal standards. See Silkwood v. Kerr-McGee Corp., 464
U. S. 238, 249 (1984).
Federal law makes a single sovereign responsible for
maintaining a comprehensive and unified system to keep
track of aliens within the Nation’s borders. If §3 of the
Arizona statute were valid, every State could give itself
independent authority to prosecute federal registration
violations, “diminish[ing] the [Federal Government]’s control
over enforcement” and “detract[ing] from the ‘integrated
scheme of regulation’ created by Congress.” Wisconsin
Dept. of Industry v. Gould Inc., 475 U. S. 282, 288–289
(1986). Even if a State may make violation of federal
law a crime in some instances, it cannot do so in a field
(like the field of alien registration) that has been occupied
by federal law. See California v. Zook, 336 U. S. 725, 730–
731, 733 (1949); see also In re Loney, 134 U. S. 372, 375–
376 (1890) (States may not impose their own punishment
for perjury in federal courts).
Arizona contends that §3 can survive preemption be­
cause the provision has the same aim as federal law and
adopts its substantive standards. This argument not only
ignores the basic premise of field preemption—that States
may not enter, in any respect, an area the Federal Gov­
ernment has reserved for itself—but also is unpersuasive

 Cite as: 567 U. S. ____ (2012)

11

Opinion of the Court

on its own terms. Permitting the State to impose its own
penalties for the federal offenses here would conflict with
the careful framework Congress adopted. Cf. Buckman
Co. v. Plaintiffs’ Legal Comm., 531 U. S. 341, 347–348
(2001) (States may not impose their own punishment for
fraud on the Food and Drug Administration); Wisconsin
Dept., supra, at 288 (States may not impose their own
punishment for repeat violations of the National Labor
Relations Act). Were §3 to come into force, the State
would have the power to bring criminal charges against
individuals for violating a federal law even in circum­
stances where federal officials in charge of the comprehen­
sive scheme determine that prosecution would frustrate
federal policies.
There is a further intrusion upon the federal scheme.
Even where federal authorities believe prosecution is appropriate, there is an inconsistency between §3 and federal law with respect to penalties. Under federal law,
the failure to carry registration papers is a misdemeanor
that may be punished by a fine, imprisonment, or a term
of probation. See 8 U. S. C. §1304(e) (2006 ed.); 18 U. S. C.
§3561. State law, by contrast, rules out probation as a
possible sentence (and also eliminates the possibility of
a pardon). See Ariz. Rev. Stat. Ann. §13–1509(D) (West
Supp. 2011). This state framework of sanctions creates a
conflict with the plan Congress put in place. See Wisconsin Dept., supra, at 286 (“[C]onflict is imminent whenever
two separate remedies are brought to bear on the same
activity” (internal quotation marks omitted)).
These specific conflicts between state and federal law
simply underscore the reason for field preemption. As it
did in Hines, the Court now concludes that, with respect
to the subject of alien registration, Congress intended to
preclude States from “complement[ing] the federal law, or
enforc[ing] additional or auxiliary regulations.” 312 U. S.,
at 66–67. Section 3 is preempted by federal law.

 12

ARIZONA v. UNITED STATES
Opinion of the Court

B
Section 5(C)
Unlike §3, which replicates federal statutory require­
ments, §5(C) enacts a state criminal prohibition where no
federal counterpart exists. The provision makes it a state
misdemeanor for “an unauthorized alien to knowingly apply for work, solicit work in a public place or perform
work as an employee or independent contractor” in Ari­
zona. Ariz. Rev. Stat. Ann. §13–2928(C) (West Supp. 2011).
Violations can be punished by a $2,500 fine and incarcera­
tion for up to six months. See §13–2928(F); see also
§§13–707(A)(1) (West 2010); 13–802(A); 13–902(A)(5). The
United States contends that the provision upsets the balance struck by the Immigration Reform and Control Act
of 1986 (IRCA) and must be preempted as an obstacle
to the federal plan of regulation and control.
When there was no comprehensive federal program
regulating the employment of unauthorized aliens, this
Court found that a State had authority to pass its own
laws on the subject. In 1971, for example, California
passed a law imposing civil penalties on the employment
of aliens who were “not entitled to lawful residence in the
United States if such employment would have an adverse
effect on lawful resident workers.” 1971 Cal. Stats. ch.
1442, §1(a). The law was upheld against a preemption
challenge in De Canas v. Bica, 424 U. S. 351 (1976). De
Canas recognized that “States possess broad authority
under their police powers to regulate the employment
relationship to protect workers within the State.” Id., at
356. At that point, however, the Federal Government had
expressed no more than “a peripheral concern with [the]
employment of illegal entrants.” Id., at 360; see Whiting,
563 U. S., at ___ (slip op., at 3).
Current federal law is substantially different from the
regime that prevailed when De Canas was decided. Con­
gress enacted IRCA as a comprehensive framework for

 Cite as: 567 U. S. ____ (2012)

13

Opinion of the Court

“combating the employment of illegal aliens.” Hoffman
Plastic Compounds, Inc. v. NLRB, 535 U. S. 137, 147
(2002). The law makes it illegal for employers to know­
ingly hire, recruit, refer, or continue to employ unauthorized
workers. See 8 U. S. C. §§1324a(a)(1)(A), (a)(2). It also
requires every employer to verify the employment authori­
zation status of prospective employees. See §§1324a(a)
(1)(B), (b); 8 CFR §274a.2(b) (2012). These requirements
are enforced through criminal penalties and an escalat­
ing series of civil penalties tied to the number of times
an employer has violated the provisions. See 8 U. S. C.
§§1324a(e)(4), (f); 8 CFR §274a.10.
This comprehensive framework does not impose federal
criminal sanctions on the employee side (i.e., penalties on
aliens who seek or engage in unauthorized work). Under
federal law some civil penalties are imposed instead. With
certain exceptions, aliens who accept unlawful employ­
ment are not eligible to have their status adjusted to
that of a lawful permanent resident. See 8 U. S. C.
§§1255(c)(2), (c)(8). Aliens also may be removed from the
country for having engaged in unauthorized work. See
§1227(a)(1)(C)(i); 8 CFR §214.1(e). In addition to specify­
ing these civil consequences, federal law makes it a crime
for unauthorized workers to obtain employment through
fraudulent means. See 18 U. S. C. §1546(b). Congress has
made clear, however, that any information employees
submit to indicate their work status “may not be used” for
purposes other than prosecution under specified federal
criminal statutes for fraud, perjury, and related conduct.
See 8 U. S. C. §§1324a(b)(5), (d)(2)(F)–(G).
The legislative background of IRCA underscores the fact
that Congress made a deliberate choice not to impose crim­

inal penalties on aliens who seek, or engage in, unauthor­
ized employment. A commission established by Congress
to study immigration policy and to make recommen­
dations concluded these penalties would be “unnecessary

 14

ARIZONA v. UNITED STATES
Opinion of the Court

and unworkable.” U. S. Immigration Policy and the Na­
tional Interest: The Final Report and Recommendations of
the Select Commission on Immigration and Refugee Policy
with Supplemental Views by Commissioners 65–66 (1981);
see Pub. L. 95–412, §4, 92 Stat. 907. Proposals to make
unauthorized work a criminal offense were debated and
discussed during the long process of drafting IRCA. See
Brief for Service Employees International Union et al. as
Amici Curiae 9–12. But Congress rejected them. See, e.g.,
119 Cong. Rec. 14184 (1973) (statement of Rep. Dennis).
In the end, IRCA’s framework reflects a considered judg­
ment that making criminals out of aliens engaged in
unauthorized work—aliens who already face the possibil­
ity of employer exploitation because of their removable
status—would be inconsistent with federal policy and objectives. See, e.g., Hearings before the Subcommittee
No. 1 of the House Committee on the Judiciary, 92d Cong.,
1st Sess., pt. 3, pp. 919–920 (1971) (statement of Rep.
Rodino, the eventual sponsor of IRCA in the House of
Representatives).
IRCA’s express preemption provision, which in most
instances bars States from imposing penalties on employ­
ers of unauthorized aliens, is silent about whether addi­
tional penalties may be imposed against the employees
themselves. See 8 U. S. C. §1324a(h)(2); Whiting, supra,
at ___–___ (slip op., at 1–2). But the existence of an “ex­
press pre-emption provisio[n] does not bar the ordinary
working of conflict pre-emption principles” or impose a
“special burden” that would make it more difficult to
establish the preemption of laws falling outside the clause.
Geier v. American Honda Motor Co., 529 U. S. 861, 869–
872 (2000); see Sprietsma v. Mercury Marine, 537 U. S. 51,
65 (2002).
The ordinary principles of preemption include the well­
settled proposition that a state law is preempted where it
“stands as an obstacle to the accomplishment and exe­

 Cite as: 567 U. S. ____ (2012)

15

Opinion of the Court

cution of the full purposes and objectives of Congress.”
Hines, 312 U. S., at 67. Under §5(C) of S. B. 1070, Arizona
law would interfere with the careful balance struck by
Congress with respect to unauthorized employment of
aliens. Although §5(C) attempts to achieve one of the
same goals as federal law—the deterrence of unlawful
employment—it involves a conflict in the method of en­
forcement. The Court has recognized that a “[c]onflict
in technique can be fully as disruptive to the system Con­
gress enacted as conflict in overt policy.” Motor Coach
Employees v. Lockridge, 403 U. S. 274, 287 (1971). The
correct instruction to draw from the text, structure, and
history of IRCA is that Congress decided it would be inap­
propriate to impose criminal penalties on aliens who seek
or engage in unauthorized employment. It follows that a
state law to the contrary is an obstacle to the regulatory
system Congress chose. See Puerto Rico Dept. of Consumer Affairs v. ISLA Petroleum Corp., 485 U. S. 495, 503
(1988) (“Where a comprehensive federal scheme intention­
ally leaves a portion of the regulated field without con­
trols, then the pre-emptive inference can be drawn—not
from federal inaction alone, but from inaction joined with
action”). Section 5(C) is preempted by federal law.
C
Section 6
Section 6 of S. B. 1070 provides that a state officer,
“without a warrant, may arrest a person if the officer has
probable cause to believe . . . [the person] has committed
any public offense that makes [him] removable from the
United States.” Ariz. Rev. Stat. Ann. §13–3883(A)(5)
(West Supp. 2011). The United States argues that arrests
authorized by this statute would be an obstacle to the
removal system Congress created.
As a general rule, it is not a crime for a removable alien
to remain present in the United States. See INS v. Lopez-

 16

ARIZONA v. UNITED STATES
Opinion of the Court

Mendoza, 468 U. S. 1032, 1038 (1984). If the police stop
someone based on nothing more than possible removabil­
ity, the usual predicate for an arrest is absent. When an
alien is suspected of being removable, a federal official
issues an administrative document called a Notice to Ap­
pear. See 8 U. S. C. §1229(a); 8 CFR §239.1(a) (2012).
The form does not authorize an arrest. Instead, it gives
the alien information about the proceedings, including the
time and date of the removal hearing. See 8 U. S. C.
§1229(a)(1). If an alien fails to appear, an in absentia
order may direct removal. §1229a(5)(A).
The federal statutory structure instructs when it is ap­
propriate to arrest an alien during the removal process.
For example, the Attorney General can exercise discretion
to issue a warrant for an alien’s arrest and detention
“pending a decision on whether the alien is to be removed
from the United States.” 8 U. S. C. §1226(a); see Memo­
randum from John Morton, Director, ICE, to All Field
Office Directors et al., Exercising Prosecutorial Discretion
Consistent with the Civil Immigration Enforcement Prior­
ities of the Agency for the Apprehension, Detention, and
Removal of Aliens (June 17, 2011) (hereinafter 2011 ICE
Memorandum) (describing factors informing this and re­
lated decisions). And if an alien is ordered removed after
a hearing, the Attorney General will issue a warrant.
See 8 CFR §241.2(a)(1). In both instances, the warrants
are executed by federal officers who have received training
in the enforcement of immigration law. See §§241.2(b),
287.5(e)(3). If no federal warrant has been issued, those
officers have more limited authority. See 8 U. S. C. §1357(a).
They may arrest an alien for being “in the United
States in violation of any [immigration] law or regulation,” for example, but only where the alien “is likely to
escape before a warrant can be obtained.” §1357(a)(2).
Section 6 attempts to provide state officers even greater
authority to arrest aliens on the basis of possible remova­

 Cite as: 567 U. S. ____ (2012)

17

Opinion of the Court

bility than Congress has given to trained federal immigration officers. Under state law, officers who believe
an alien is removable by reason of some “public offense”
would have the power to conduct an arrest on that basis
regardless of whether a federal warrant has issued or
the alien is likely to escape. This state authority could be
exercised without any input from the Federal Government
about whether an arrest is warranted in a particular case.
This would allow the State to achieve its own immigra­
tion policy. The result could be unnecessary harassment
of some aliens (for instance, a veteran, college student, or
someone assisting with a criminal investigation) whom
federal officials determine should not be removed.
This is not the system Congress created. Federal law
specifies limited circumstances in which state officers may
perform the functions of an immigration officer. A princi­
pal example is when the Attorney General has granted
that authority to specific officers in a formal agreement
with a state or local government. See §1357(g)(1); see also
§1103(a)(10) (authority may be extended in the event of an
“imminent mass influx of aliens off the coast of the United
States”); §1252c (authority to arrest in specific circum­
stance after consultation with the Federal Government);
§1324(c) (authority to arrest for bringing in and harboring
certain aliens). Officers covered by these agreements are
subject to the Attorney General’s direction and super­
vision. §1357(g)(3). There are significant complexities
involved in enforcing federal immigration law, including
the determination whether a person is removable. See
Padilla v. Kentucky, 559 U. S. ___, ___–___ (2010) (ALITO,
J., concurring in judgment) (slip op., at 4–7). As a result,
the agreements reached with the Attorney General must
contain written certification that officers have received
adequate training to carry out the duties of an immigra­
tion officer. See §1357(g)(2); cf. 8 CFR §§287.5(c) (arrest power contingent on training), 287.1(g) (defining the

 18

ARIZONA v. UNITED STATES
Opinion of the Court

training).
By authorizing state officers to decide whether an
alien should be detained for being removable, §6 violates
the principle that the removal process is entrusted to the
discretion of the Federal Government. See, e.g., Reno v.
American-Arab Anti-Discrimination Comm., 525 U. S. 471,
483–484 (1999); see also Brief for Former INS Commis­
sioners 8–13. A decision on removability requires a de­
termination whether it is appropriate to allow a foreign
national to continue living in the United States. Decisions
of this nature touch on foreign relations and must be made
with one voice. See Jama v. Immigration and Customs
Enforcement, 543 U. S. 335, 348 (2005) (“Removal deci­
sions, including the selection of a removed alien’s destina­
tion, may implicate [the Nation’s] relations with foreign
powers and require consideration of changing political and
economic circumstances” (internal quotation marks omit­
ted)); see also Galvan v. Press, 347 U. S. 522, 531 (1954)
(“Policies pertaining to the entry of aliens and their right
to remain here are . . . entrusted exclusively to Congress
. . .”); Truax v. Raich, 239 U. S. 33, 42 (1915) (“The author­
ity to control immigration—to admit or exclude aliens—is
vested solely in the Federal Government”).
In defense of §6, Arizona notes a federal statute permit­
ting state officers to “cooperate with the Attorney General
in the identification, apprehension, detention, or removal
of aliens not lawfully present in the United States.” 8
U. S. C. §1357(g)(10)(B). There may be some ambiguity as
to what constitutes cooperation under the federal law; but
no coherent understanding of the term would incorporate
the unilateral decision of state officers to arrest an alien
for being removable absent any request, approval, or other
instruction from the Federal Government. The Depart­
ment of Homeland Security gives examples of what would
constitute cooperation under federal law. These include
situations where States participate in a joint task force

 Cite as: 567 U. S. ____ (2012)

19

Opinion of the Court

with federal officers, provide operational support in exe­
cuting a warrant, or allow federal immigration officials
to gain access to detainees held in state facilities. See
Dept. of Homeland Security, Guidance on State and Local
Governments’ Assistance in Immigration Enforcement
and Related Matters 13–14 (2011), online at http://
www.dhs.gov/files/resources/immigration.shtm (all Inter­
net materials as visited June 21, 2012, and available in
Clerk of Court’s case file). State officials can also assist
the Federal Government by responding to requests for
information about when an alien will be released from
their custody. See §1357(d). But the unilateral state
action to detain authorized by §6 goes far beyond these
measures, defeating any need for real cooperation.
Congress has put in place a system in which state offic­
ers may not make warrantless arrests of aliens based on
possible removability except in specific, limited circum­
stances. By nonetheless authorizing state and local officers to engage in these enforcement activities as a general
matter, §6 creates an obstacle to the full purposes and
objectives of Congress. See Hines, 312 U. S., at 67. Sec­
tion 6 is preempted by federal law.
D
Section 2(B)
Section 2(B) of S. B. 1070 requires state officers to make
a “reasonable attempt . . . to determine the immigration
status” of any person they stop, detain, or arrest on some
other legitimate basis if “reasonable suspicion exists that
the person is an alien and is unlawfully present in the
United States.” Ariz. Rev. Stat. Ann. §11–1051(B) (West
2012). The law also provides that “[a]ny person who is
arrested shall have the person’s immigration status de­
termined before the person is released.” Ibid. The accepted
way to perform these status checks is to contact ICE,
which maintains a database of immigration records.

 20

ARIZONA v. UNITED STATES
Opinion of the Court

Three limits are built into the state provision. First, a
detainee is presumed not to be an alien unlawfully present
in the United States if he or she provides a valid Arizona
driver’s license or similar identification. Second, officers
“may not consider race, color or national origin . . . except
to the extent permitted by the United States [and] Ari­
zona Constitution[s].” Ibid. Third, the provisions must
be “implemented in a manner consistent with federal law
regulating immigration, protecting the civil rights of all
persons and respecting the privileges and immunities of
United States citizens.” §11–1051(L) (West 2012).
The United States and its amici contend that, even with
these limits, the State’s verification requirements pose an
obstacle to the framework Congress put in place. The first
concern is the mandatory nature of the status checks. The
second is the possibility of prolonged detention while the
checks are being performed.
1
Consultation between federal and state officials is an
important feature of the immigration system. Congress
has made clear that no formal agreement or special train­
ing needs to be in place for state officers to “communicate
with the [Federal Government] regarding the immigration
status of any individual, including reporting knowledge
that a particular alien is not lawfully present in the United
States.” 8 U. S. C. §1357(g)(10)(A). And Congress has
obligated ICE to respond to any request made by state
officials for verification of a person’s citizenship or immigration status. See §1373(c); see also §1226(d)(1)(A)
(requiring a system for determining whether individuals
arrested for aggravated felonies are aliens). ICE’s Law
Enforcement Support Center operates “24 hours a day,
seven days a week, 365 days a year” and provides, among
other things, “immigration status, identity information
and real-time assistance to local, state and federal law

 Cite as: 567 U. S. ____ (2012)

21

Opinion of the Court

enforcement agencies.” ICE, Fact Sheet: Law Enforce­
ment Support Center (May 29, 2012), online at http://
www.ice.gov/news/library/factsheets/lesc.htm.
LESC re­

sponded to more than one million requests for information
in 2009 alone. App. 93.
The United States argues that making status verifica­
tion mandatory interferes with the federal immigration
scheme. It is true that §2(B) does not allow state officers
to consider federal enforcement priorities in deciding
whether to contact ICE about someone they have de­
tained. See Brief for United States 47–50. In other words,
the officers must make an inquiry even in cases where it
seems unlikely that the Attorney General would have
the alien removed. This might be the case, for example,
when an alien is an elderly veteran with significant and
longstanding ties to the community. See 2011 ICE Memo­
randum 4–5 (mentioning these factors as relevant).
Congress has done nothing to suggest it is inappropriate
to communicate with ICE in these situations, however.
Indeed, it has encouraged the sharing of information about
possible immigration violations. See 8 U. S. C. §1357(g)
(10)(A). A federal statute regulating the public benefits
provided to qualified aliens in fact instructs that “no State
or local government entity may be prohibited, or in any
way restricted, from sending to or receiving from [ICE]
information regarding the immigration status, lawful or
unlawful, of an alien in the United States.” §1644. The
federal scheme thus leaves room for a policy requiring
state officials to contact ICE as a routine matter. Cf.
Whiting, 563 U. S., at ___–___ (slip op., at 23–24) (reject­
ing argument that federal law preempted Arizona’s re­
quirement that employers determine whether employees
were eligible to work through the federal E-Verify system
where the Federal Government had encouraged its use).

 22

ARIZONA v. UNITED STATES
Opinion of the Court

2
Some who support the challenge to §2(B) argue that, in
practice, state officers will be required to delay the release
of some detainees for no reason other than to verify their
immigration status. See, e.g., Brief for Former Arizona
Attorney General Terry Goddard et al. as Amici Curiae 37,
n. 49. Detaining individuals solely to verify their immi­
gration status would raise constitutional concerns. See,
e.g., Arizona v. Johnson, 555 U. S. 323, 333 (2009); Illinois
v. Caballes, 543 U. S. 405, 407 (2005) (“A seizure that is
justified solely by the interest in issuing a warning ticket
to the driver can become unlawful if it is prolonged beyond
the time reasonably required to complete that mission”).
And it would disrupt the federal framework to put state
officers in the position of holding aliens in custody for
possible unlawful presence without federal direction and
supervision. Cf. Part IV–C, supra (concluding that Ari­
zona may not authorize warrantless arrests on the basis of
removability). The program put in place by Congress does
not allow state or local officers to adopt this enforcement
mechanism.
But §2(B) could be read to avoid these concerns. To take
one example, a person might be stopped for jaywalking in
Tucson and be unable to produce identification. The first
sentence of §2(B) instructs officers to make a “reasonable”
attempt to verify his immigration status with ICE if there
is reasonable suspicion that his presence in the United
States is unlawful. The state courts may conclude that,
unless the person continues to be suspected of some crime
for which he may be detained by state officers, it would
not be reasonable to prolong the stop for the immigration
inquiry. See Reply Brief for Petitioners 12, n. 4 (“[Section
2(B)] does not require the verification be completed during
the stop or detention if that is not reasonable or practica­
ble”); cf. Muehler v. Mena, 544 U. S. 93, 101 (2005) (finding
no Fourth Amendment violation where questioning about

 Cite as: 567 U. S. ____ (2012)

23

Opinion of the Court

immigration status did not prolong a stop).
To take another example, a person might be held pend­
ing release on a charge of driving under the influence of
alcohol. As this goes beyond a mere stop, the arrestee
(unlike the jaywalker) would appear to be subject to the
categorical requirement in the second sentence of §2(B)
that “[a]ny person who is arrested shall have the person’s
immigration status determined before [he] is released.”
State courts may read this as an instruction to initiate a
status check every time someone is arrested, or in some
subset of those cases, rather than as a command to hold
the person until the check is complete no matter the cir­
cumstances. Even if the law is read as an instruction to
complete a check while the person is in custody, moreover,
it is not clear at this stage and on this record that the
verification process would result in prolonged detention.
However the law is interpreted, if §2(B) only requires
state officers to conduct a status check during the course
of an authorized, lawful detention or after a detainee has
been released, the provision likely would survive preemption—at least absent some showing that it has other
consequences that are adverse to federal law and its objec­
tives. There is no need in this case to address whether
reasonable suspicion of illegal entry or another immigra­
tion crime would be a legitimate basis for prolonging a
detention, or whether this too would be preempted by
federal law. See, e.g., United States v. Di Re, 332 U. S.
581, 589 (1948) (authority of state officers to make arrests
for federal crimes is, absent federal statutory instruction,
a matter of state law); Gonzales v. Peoria, 722 F. 2d 468,
475–476 (CA9 1983) (concluding that Arizona officers have
authority to enforce the criminal provisions of federal
immigration law), overruled on other grounds in HodgersDurgin v. de la Vina, 199 F. 3d 1037 (CA9 1999).
The nature and timing of this case counsel caution in
evaluating the validity of §2(B). The Federal Government

 24

ARIZONA v. UNITED STATES
Opinion of the Court

has brought suit against a sovereign State to challenge the
provision even before the law has gone into effect. There
is a basic uncertainty about what the law means and how
it will be enforced. At this stage, without the benefit of a
definitive interpretation from the state courts, it would be
inappropriate to assume §2(B) will be construed in a way
that creates a conflict with federal law. Cf. Fox v. Washington, 236 U. S. 273, 277 (1915) (“So far as statutes fairly
may be construed in such a way as to avoid doubtful con­
stitutional questions they should be so construed; and it is
to be presumed that state laws will be construed in that
way by the state courts” (citation omitted)). As a result,
the United States cannot prevail in its current challenge.
See Huron Portland Cement Co. v. Detroit, 362 U. S. 440,
446 (1960) (“To hold otherwise would be to ignore the
teaching of this Court’s decisions which enjoin seeking out
conflicts between state and federal regulation where none
clearly exists”). This opinion does not foreclose other
preemption and constitutional challenges to the law as
interpreted and applied after it goes into effect.
V
Immigration policy shapes the destiny of the Nation.
On May 24, 2012, at one of this Nation’s most distin­
guished museums of history, a dozen immigrants stood
before the tattered flag that inspired Francis Scott Key
to write the National Anthem. There they took the oath
to become American citizens. The Smithsonian, News
Release, Smithsonian Citizenship Ceremony Welcomes
a Dozen New Americans (May 24, 2012), online at
http://newsdesk.si.edu/releases. These naturalization ceremonies bring together men and women of different ori­
gins who now share a common destiny. They swear a
common oath to renounce fidelity to foreign princes, to
defend the Constitution, and to bear arms on behalf of the
country when required by law. 8 CFR §337.1(a) (2012).

 Cite as: 567 U. S. ____ (2012)

25

Opinion of the Court

The history of the United States is in part made of the
stories, talents, and lasting contributions of those who
crossed oceans and deserts to come here.
The National Government has significant power to
regulate immigration. With power comes responsibility,
and the sound exercise of national power over immigration
depends on the Nation’s meeting its responsibility to base
its laws on a political will informed by searching, thought­
ful, rational civic discourse. Arizona may have under­
standable frustrations with the problems caused by illegal
immigration while that process continues, but the State
may not pursue policies that undermine federal law.
*
*
*
The United States has established that §§3, 5(C), and 6
of S. B. 1070 are preempted. It was improper, however, to
enjoin §2(B) before the state courts had an opportunity to
construe it and without some showing that enforcement of
the provision in fact conflicts with federal immigration law
and its objectives.
The judgment of the Court of Appeals for the Ninth
Circuit is affirmed in part and reversed in part. The case
is remanded for further proceedings consistent with this
opinion.
It is so ordered.
JUSTICE KAGAN took no part in the consideration or
decision of this case.