Like others here said, it's extremely unlikely an enemy would land sufficient soldiers to occupy America. If they did and assuming the official armed forces had been defeated (also very unlikely), the enemy could expect significant armed guerilla resistance.
That said, I think the appeal for Americans watching Red Dawn is that it's a fantasy where we get to use deadly force against our enemies and be completely justified. They'll tell you all our wars are necessary and moral, but our last "just war" that was even a little cut and dry was world war two. (And still plenty of ethical questions even there)
But here's a daydream where the bad guy has attacked us and imprisoned our families and it's time for payback!
As an aside I think that's the gimmick of every Quentin Tarantino movie: someone else did something horrible so now it's ok that I'm about to fuck them up in a gruesome way. (Kill Bill, Inglorious Bastards, Django Unchained etc)