[jltsiren]

US states are not fully sovereign. There is a simple test for that: can a state secede unilaterally. If not, states are subordinate to the federal government.

The original idea of the US was a decentralized state. The Union is the sovereign entity, while power rests with the individual states, unless explicitly given to the federal government. Over time, it became clear that you can't run a country like that in the modern world. Slowly, with creative interpretations of the Constitution, the federal government gained more and more power over the states.

One of the distinctive features of the actual US system is that it's often unclear which level of government has the authority to regulate a particular matter. Much of the political debate is about the constitutionality of laws and regulations, which may have been in force for decades. And controversial legislation often ends up in the Supreme Court, which may then decide that the state or the federal government did not have the authority to regulate that.