Writing for the Court, Justice Anthony Kennedy began by establishing the “well settled” principle that the federal government has “broad, undoubted power” over the issue of immigration. This power, Kennedy explained, originated in the Constitution’s express and inherent grants of authority to the federal government.  

First, the Constitution assigns to the federal government the responsibility to “establish an uniform Rule of Naturalization.” Second, the Constitution grants the federal government “inherent power as sovereign to control and conduct relations with foreign nations.” Kennedy explained that foreign countries with concerns regarding “the status, safety, and security of their nationals” in the U.S. must be able to communicate with one national entity, not the 50 separate states. He warned that “[p]erceived mistreatment of aliens in the United States may lead to harmful reciprocal treatment of American citizens abroad.” Further, Kennedy argued that discretion in immigration enforcement “embraces immediate human concerns” and involves policy decisions and vary on a case-by-case basis. Thus, “[t]he dynamic nature of relations with other countries requires the Executive Branch to ensure that enforcement policies are consistent with this Nation’s foreign policy.” 

Before evaluating the provisions of SB 1070, Kennedy noted that “[t]he pervasiveness of federal regulation does not diminish the importance of immigration policy to the States,” and acknowledged that Arizona faces the brunt of much of the consequences that stem from illegal immigration. 

Regarding Section 2(B), Kennedy established that consultation between federal and state officials is an important feature of the immigration system. Kennedy pointed out that Congress doesn’t require a formal agreement to allow communication between state and federal officers regarding the immigration status of individuals, and has actually obligated ICE to respond to any request from a state official for verification of an individual’s citizenship or immigration status. Further, he noted that Congress has done nothing to suggest it is inappropriate to communicate with ICE in these situations,” and has “encouraged the sharing of information” regarding potential immigration violations. Thus, Kennedy found that federal law “leaves room for a policy requiring state officials to contact ICE as a routine matter.” Kennedy added that the Court’s holding “does not foreclose other preemption and constitutional challenges to the law as interpreted and applied after it goes into effect.” 

Moving to Section 3, Kennedy referred to the Court’s decision in Hines v. Davidowitz (1941) in establishing that states may not “curtail or complement” federal law or “enforce additional or auxiliary regulations.” Regarding the specific framework constructed by section 3, Kennedy found that it was unconstitutional because Congress had already enacted a full set of standards to govern alien registration and punishments for non-compliance. He explained that “[w]here Congress occupies an entire field, as it has in the field of alien registration, even complementary state regulation is impermissible. Field preemption reflects a congressional decision to foreclose any state regulation in the area, even if it is parallel to federal standards.” Responding to Arizona’s argument that Section 3 has the same aim as federal law, Kennedy stated that it “not only ignores the basic premise of field preemption—that States may not enter, in any respect, an area the Federal Government has reserved for itself—but also is unpersuasive on its own terms.” Kennedy concluded that Section 3 was preempted by federal law because “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.’”  

Evaluating Section 5(c) next, Kennedy explained that Congress has already established comprehensive framework under IRCA to combat the employment of illegal aliens. Under the IRCA, employers may be criminally liable for knowingly hiring, recruiting, or employing unauthorized workers, but no similar criminal sanctions were imposed against employees. Kennedy explained that the legislative history of the IRCA “underscores the fact that Congress made a deliberate choice not to impose criminal penalties on aliens who seek, or engage in, unauthorized employment,” and its framework “reflects a considered judgment that making criminals out of aliens engaged in unauthorized work—aliens who already face the possibility of employer exploitation because of their removable status—would be inconsistent with federal policy and objectives.” Kennedy established that “[t]he 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 execution of the full purposes and objectives of Congress,’” and concluded that Section 5(c) was preempted by federal law since it “would interfere with the careful balance struck by Congress.”  

Lastly, evaluating Section 6, Kennedy first established that “[a]s a general rule, it is not a crime for a removable alien to remain present in the United States.” Rather, aliens suspected of being removable are given a Notice to Appear for a removal hearing, only after which they may be deported. Kennedy found that Section 6 was an attempt to give “state officers even greater authority to arrest aliens on the basis of possible removability than Congress has given to trained federal immigration officers.” He noted that the authority granted by Section 6 would allow state officers to act without any input from the federal government about whether an arrest is warranted, which “would allow the State to achieve its own immigration policy.” Kennedy argued that this is not the system created by Congress, and that “[b]y 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.” He concluded that Section 6 was preempted by federal law, writing that “[a] decision on removability requires a determination 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.” 

In closing, Kennedy wrote that “[t]he 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, thoughtful, rational civic discourse.” Kennedy ultimately concluded that while “Arizona may have understandable frustrations with the problems caused by illegal immigration... the State may not pursue policies that undermine federal law.”