The United States Supreme Court held that a Pennsylvania state system of alien registration was superseded by a federal system (the Alien Registration Act) because it was an "obstacle to the accomplishment" of its goals.

Pennsylvania passed a statute requiring aliens to register with the state, carry a state-issued identification card, and pay a small registration fee.

[clarification needed] [citation needed] The Court applied the prong of preemption doctrine which inquires whether state "law stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress.

Justice Hugo L. Black emphasized the supremacy of federal power over this area of law:[2] That the supremacy of the national power in the general field of foreign affairs, including power over immigration, naturalization and deportation, is made clear by the Constitution, was pointed out by the authors of The Federalist in 1787, and has since been given continuous recognition by this Court.

The Federal Government, representing as it does the collective interests of the forty-eight states, is entrusted with full and exclusive responsibility for the conduct of affairs with foreign sovereignties.