Americans would definitely put the 2nd amendment to use if an invading army were present. The US also fought wars on American soil against the British twice. On both occasions the British took control of the major American cities but were unable to end the war.

On top of that you have a frontier society that has long used the gun in Western settlement, most notably with the various wars against Indian tribes. And while I don't buy into the nationalist bullshit that a lot of people in the US promote, I do like Stephen Fry's commentary about the largest section in every American bookstore being self-help and the US has a very strong can-do attitude in its culture.