Arizona v. United States, Monday’s Supreme Court opinion striking down most of an Arizona law that strengthened immigration enforcement, is largely being seen through the lens of federalism. On this view, the Supreme Court has confirmed that the national government takes the lead on immigration policy and the states must fall into line. In fact, the ruling matters more for what it says about the advance of executive power in the United States, especially in connection with President Obama’s decision not to enforce immigration laws against certain young illegal immigrants. The imperial presidency, much criticized by Democrats during the Bush administration and by Republicans during the current administration for its dominance over security and war policy, now prevails in the immigration arena as well.

In 2010, Arizona enacted a law, SB 1070, that required state police officers to check the immigration status of a person whom they stop, detain, or arrest—and to arrest anyone who has committed a crime that means he can be deported under federal law. The Arizona law also made two other federal immigration violations crimes under state law. The court struck down all but the first of these provisions—leaving in place only the status checking part—essentially on the grounds that the rest of Arizona’s plans conflicted with the federal scheme for regulating immigration. Under the Supremacy Clause of the U.S. Constitution, state law that conflicts with federal law is invalid.

Arizona argued, unsuccessfully for the most part, that its laws did not conflict with federal law but enhanced it. The state reasoned that the purpose of federal immigration law is to regulate the flow of migrants and foreign workers. When people enter the United States in violation of the rules, the U.S. government is supposed to detain and remove them. But the U.S. government failed to stanch the flow into Arizona because of limited resources, and possibly political pressure from Hispanic groups. Arizona, bearing much of the cost of illegal immigration, wanted stronger enforcement for itself.

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