Well, despite oft quoted Japanese Admiral, I think the main reason we haven't had a "successful" invasion is that we have Canada to the north of us and Mexico to the south neither of which is a particularly militaristic country and we have a couple thousand miles of open ocean to the east and west of us. It took a pretty impressive act of military planning to attack Pearl Harbor with anything approaching surprise and that attack just destroyed a lot of ships and facilities. There was no effort to take and hold Hawaii. To land a several million man invasion force on the Californian or Carolina coast and keep the supply lines open is somewhere in the realm of the nonsensical. Even the British as a superpower couldn't successfully maintain an occupying army in 1776 or and invading army in 1812. The strategic and tactical considerations of invading the United States are insurmountable. You could of course land in South America and march up through the rain forest, but that presents its own equally difficult problems. Personally owned firearms don't even begin to factor into the equation.

The forces that the second amendment protects our freedom against is our government and our fellow citizens, not a foreign power.