Well, the duty of the courts is to settle disputes over the laws, and the Constitution states that the Constitution is the supreme law of the land, followed by treaties, followed by federal laws (in their proper jurisdiction), followed by state laws, and municipal laws are at the bottom. If two laws conflict, the higher one has priority and the lower is unenforceable.
The problem with this claim, with respect to Missouri v. Holland (1920), is that the claim of Missouri was that the state’s rights under the Tenth Amendment were being violated. The Tenth Amendment, having been passed after the rest of the Constitution, takes precedence over Art. VI, in the same way that the 13th Amendment abolition of slavery takes precedence over Art. IV, sec. 2, which guarantees the return of persons “held to Service or Labour in one State.”
