They might be able set off a dirty bomb in the docks in California. Wait for it to cause chaos then attack from the air and invade by land, but I don't think they would ever be able to put down all the resistance and hold it. The South would pose the most militia threat. I live on the East Coast, and the Appalachian offers a sturdy defense against ground troops. I know the hills well, and could conceivably survive the length of an invasion if need be. Many in this part of the country can make the same claim. Some have some crazy weapon stashes and the know how to construct formidable means of defense and offense. Then you have the miltary bases and large military personel, and when you factor in the sheer size of the country I don't see a ground invasion being practical. Nuclear weapons and bombs would be the only way, but for that we have our missle defense shields which would need to be taken offline.