The national guard, the modern militia, can still be called out by state governors. Under the law they are controlled by the federal government, but in a dictat

The national guard, the modern militia, can still be called out by state governors. Under the law they are controlled by the federal government, but in a dictatorship and civil war situation that might not mean much (just as how in the American civil war many members of the military resigned and fought for the southern rebels) It’s likely any successful revolution or insurgency would have outside backing. Vietnam was backed by the USSR, the taliban receives support from Pakistan, the insurgency in Iraq was supported by Iran, and so on. Just like we ourselves destabilized Syria and Libya by supporting insurgencies there. As demonstrated by the 2016 elections, there are other countries out there even now who are eager to interfere with the US. In a theoretical future US dictatorship, perhaps support for an insurgency might come over the border from Canada and Mexico. We are dealing with a hypothetical situation far from what today’s international and national politics look like, of course.