Several news stories have led me to believe Americans don't really understand that American law has a fairly strict order of precedence. A state can't overturn a federal law. The president--nor even Congress--can't overturn a ratified treaty.
As a refresher, this is the order of precedence:
US Constitution
Treaties
Federal statute
Federal regulations
Federal caselaw
State Constitutions
other state laws
An authority lower on this list cannot overrule any authority higher on this list unless explicitly authorized to do so by something even higher on the list. Treaties are actually one of the highest forms of American law, and the only legal way out of a treaty obligation is to negotiate and ratify another treaty (well, I suppose a constitutional amendment is another option). Violations of treaties tend to have relatively minor consequences so we do it often, but make no mistake--that is totally illegal.
Also, you'll note I did not rank city, township, and county laws and regulations on the list. That's because, from the perspective of the US constitution, none of those exist. The US constitution creates states (and the District of Columbia), and all other local laws and regulations are expressions of the powers it delegates to the states.