Answer:
The supremacy clause (article 4, section 2 of the constitution), establishes that federal law, supreme court decisions, and US treaties — i.e., anything decided on a national level — supersedes anything decided on a state or local level. The point is to make sure that states do not subvert national interests. For instance, if the US goes to war with another nation and sets up a trade embargo, and individual state should not be allowed to pass laws which allow it to trade with the enemy; that would damage the ability of the nation to fight its war.
Step-by-step explanation: