The US is too geographically isolated for standard military assault. An enemy army would have to attack from one (or more) of four vectors...Canada, Mexico, Pacific Coast, Atlantic Coast.
Given our relatively amicable trade and political relations (and binding treaties) with both Canada and Mexico an attack from these vectors is unlikely at best. Mexico and Canada would also be reluctant to support a guerrilla war based from their countries, as the US (as usual) would crack down on the Mexican and Canadian governments because they are more visible targets than hidden insurgents.
This leaves amphibious and air assaults on our coasts. Our spy satellite network, Coast Guard, and Naval patrols would detect the movement of the massive number of transports required for any significant landing. An enemy would have to employ stealth submarine transports to get their boots on the ground for a preliminary assault.
A significant aerial assault would require multiple aircraft carriers, which would be extremely hard if not impossible to stealth. Additionally the airplanes would also need stealth technology to avoid anti-aircraft measures. The aerial assault plan would have to cripple most (preferably all) communications, command and control, and military radar emplacements on the coastline. Secondary targets would be civilian radar and communications points.
If communications could not be suppressed, the invader would be very quickly facing the combined firepower of all NATO's forces. The weight of this military retribution would crush the invasion within 30-60 days at best. US citizens would hunt down enemy soldiers personally, and every gun nut in America would jump in their SUV's and raise some hell for the enemy.
Bottom line...the US is not vulnerable to standard military invasion. It would require a highly stealthy, highly organized, and rapid strike from a technologically superior enemy. We are vulnerable to nuclear assault to some extent...but nuclear weapons don't capture territory, and their use would doom the offending party (or the nation they come from) to extreme sanctions and US occupation for a long time.