Well, hold on to it, probably not, it would be like the Iraq situation where there were armed resistance everywhere, especially with your point that you made about 2nd amend. But assuming there was a problem with our response time concerning the military, the general population here is not equiped mentally or physically to handle an effective resistance. Besides, we already have a semi-military groups that would be everywhere, ie: police, SWAT, ATF. Not to mention, the US is a pretty good size country, so holding anything that size would be difficult, just based on coverage.
When we have people complaining and suing McDonalds for making them fat, how to you think they would react to taking and dodging a bullet? I suppowse some of the street gangs that are used to it from one another might stand a chance, but still, the vast and far majority do not exercise their 2nd amend. right and the ones who do, the majority of them wouldn't be of real use either.
So I would say invade, yes, it would be possible, but to effectively to do it, no.