We haven't had a war inside our boundaries since the Civil War unless you want to count the various Indian wars that carried on into the 20th century. That means the American public doesn't have any memory of what hell war actually is. We had skirmishes with Canada when it was still a colony same as us. And then there was the war with Mexico where we ended up acquiring parts of California, Arizona, New Mexico, Oklahoma, and Texas. We have been very sheltered for all of our existence as a country. With rockets armed with nukes, we are no longer safe from countries like Iran who don't care about mutually assured destruction because they are quite happy to meet their maker since he comes with 72 virgins.