Even if another country wanted to swoop in, that is specifically what the American nuclear arsenal is there to prevent. No one's gonna eat nukes for America's benefit.
At most, other countries could try to smuggle arms across the Canadian or Mexican border to help in a civil war – but Canada and Mexico might be reluctant to participate given that they might get dragged into the war. Also, it's pretty damn hard to smuggle large quantities of gear without being noticed.