When can federal law override a conflicting state law?

In the United States, federal law overrides conflicting state law because the Constitution, federal treaties, and acts of Congress are considered the supreme law of the land.1

When a state statute contravenes federal constitutional or statutory law, it is deemed unenforceable. This legal principle is known as preemption, a doctrine under which federal law supersedes state law in the event of a conflict. Even in the absence of an express provision for preemption, state law must yield to federal acts in various circumstances.12

Conflicts between state and national government are resolved by the courts, which are obligated to refuse to enforce state statutes that conflict with federal authority. The Supreme Court of the United States serves as the final arbiter in determining whether state legislation conflicts with federal power or the Constitution. While the Tenth Amendment reserves certain powers to the states, it does not grant them the authority to maintain laws that conflict with the established powers of the federal government.13