Because we have something worth defending.
Because our culture is superior to every other one on the face of the earth. More freedom, more liberty, more ability to trancend class based on your own achivement, more chance to invent a change that will alter the world and make you rich. Because we have had the Bomb for over fifty years, and have only used it twice, and that saved millions of lives on both sides. Because we have never flown airplanes into skyscrapers to make a political point. And we are all of the above...because we have the means to prevent other nations from taking it, and up until recently, have had the backbone to use it.