With due respect to many of the answers, I believe many do not accept the spirit of the question, saying instead that it is impossible for the U.S. to be invaded. read more
The United States--as opposed to outlying territories of the United States--can't be invaded successfully. The United States is too well-armed at all levels, too overwhelmingly dominant in North America, and located in a continent sufficiently removed from military powers that might be contenders, to be invaded successfully. read more