What happens if a state or local government writes a law which conflicts with the U. S. Constitution or a federal law?

The Supremacy Clause is a clause within Article VI of the U. S. Constitution which dictates that federal law is the "supreme law of the land". This means that judges in every state must follow the Constitution, laws, and treatises of the federal government in matters which are directly or indirectly within the government's control. Under the doctrine of preemption, which is based on the Supremacy Clause, federal law preempts state law, even when the laws conflict. Thus, a federal court may require a state to stop certain behavior it believes interferes with, or is in conflict with, federal law.