What is the Supremacy Clause?

The Supremacy Clause, Article VI, Clause 2, states that “This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made... under the Authority of the United States, shall be the supreme 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.” In practice, this is the textual basis for federal preemption: when validly enacted federal law and state law genuinely conflict, state law yields. McCulloch v. Maryland (1819) gave the clause its first major application, striking down a Maryland tax on the federally chartered Bank of the United States; Chief Justice Marshall reasoned that “the power to tax involves the power to destroy,” and a state cannot use its own laws to burden or obstruct a constitutional exercise of federal power. Arizona v. United States (2012) shows the doctrine still working two centuries later, in a very different context: the Court struck down most of an Arizona immigration-enforcement law, holding that “the Federal Government has occupied the field of alien registration” and that even a complementary state law was preempted where Congress had claimed exclusive authority. The Court described preemption as arising in at least two ways beyond explicit federal statutes saying so — when a state law regulates a field Congress has claimed exclusively, or when it conflicts with or obstructs a federal law's purpose. The Supremacy Clause is distinct from the Necessary and Proper Clause, which asks what Congress may do; this clause instead settles what happens when a state tries to do something incompatible with what Congress has already validly done.