Subject: Re: HCR Gives a History Lesson
Is This explicitly stated somewhere in the Constitution, or is it only true because it wasn't stated that the states ARE required to enforce federal law?

Both. It's not an enumerated power and the Tenth Amendment says that the States have all powers not granted to the feds. Plus, the courts have also grounded it in the very structure of the government. Most countries have both a national government and divide up into regional governments. Unlike virtually every other nation, in the U.S. the national government was formed by the states, rather than other way around. So there's also more of a structural argument against the notion that states are instrumentalities of the federal government the way that cities are instrumentalities of the states.