So, just to clarify my "invasion" comment. Some count the revolutionary war as an invasion, but I don't think that's right technically. We were British colonies before we kicked their butts.
The war of 1812 was, an invasion, so yes.
Some call the Civil war an invasion. Not sure I agree with that.
Pancho Villa.. okay maybe.
And if you want to call terrorist attacks on our soil an invasion, well okay, we can split hairs on that.
Really referring to an all out invasion of ground forces onto the mainland in modern era. That's likely not to happen, one because of technology, and two because our enemies know that guerrilla warfare would be waged for decades.
Americans have the right and advantage of being armed - unlike the citizens of other countries whose governments are afraid to trust the people with arms.
-- James Madison, The Federalist Papers