Thank You,SirBeing a military dude, I have to say that the amount of guns in America and their effectiveness would largely depend on the type of occupation. Someone coming in just to destroy us would do so quite easily. An Army willing to level houses and willing to have large amounts of civilian casualties would destroy even a well trained band of citizens.
On the other hand, if it was a war like Iraq where you want to subjecticate (not saying that was the point there) and win hearts and minds, then the civilian could become dangerous if they chose to stand up. Clearing houses and cities is a pain and only grunts can do it in this type of war. You will take massive casualties if Americans are willing to die for their freedom. That then becomes the question though....How many people would stand up and fight versus just waiting to see what happened?
I think the reality is that no one really knows what it would be like to invade America because it hasn't happened. If it makes other countries afraid...great. It makes my life easier. I don't want to fight on my own soil.