I think that the U.S. navy should be able to repel any sea based invasion, especially near the coast under land and carrier based suppport. Not just that, our ships would be closer to home and easier to resupply.
Failing this and if we are invaded; I feel sorry for them. After being in a democracy and having our own freedom for so long, I would hate to try to take it away.
I think the initial assaults would be brutal, and the resistance immensly strong. As far as militia resupply goes, I would probably just strip weapons and ammo off of the enemy. Much as the french did during WWII.
Beyond this, dont we have tactical nukes for ground assaults that we would use in a 'worst case' scenario? What would constitue a worst case?
Qui tacet consentire.
Igitur qui desiderat pacem, praeparet bellum.
Nec Aspera Terrent.
Vir sapit qui pauca loquitur.