I have to start this entry with a truism: A large percentage of the American population loves spectator sports. No kidding, right? Like any other phenomenon present in the human animal,there must be some sort of physiological or evolutionary reason for this, some reason why its part of human nature. Right now, I am much to busy with work and school to do thorough research on the subject, but I am guessing it has something to do with whatever attribute present within humans that accounts for social bonding. To survive in prehistoric times, humans had to gather into tribes to for mutual protection and support. The post-industrial world we live in today is much different than the world the human organism evolved to thrive in. So today, to loosely paraphrase the unibomber, people must undertake surrogate actions to make their lives feel complete. Spectator sports seems to be the perfect substitute for primitive tribal behavior. The spectator rallies around a given team, usually one from their area, be it their city or local high school, and roots for them to win, just like primitives rooting for their tribe to be victorious against another tribe or kill a saber-tooth tiger.
War can satiates this need as well, not in the sense of actually participating in war, but in the sense of being a war spectator and rooting for your chosen side.
Rooting for the military in war, your team, make the human animal feel complete on some level. This, in itself, is neither a bad or a good thing. Throughout history, people have rooted for many militaristic just causes; the Second World War come to mind.
The dangerous thing about this facet of human behavior, is it can be used, like any other facet of behavior, to further the goals of "social exploiters." By social exploiter I basically mean the advertiser (and not just the advertiser of products; what are the people that promote presidential candidates and wars if they are not advertisers? I propose this is all they really are.)
So now that war has been initiated by whomever for whatever purpose. Since its our team against their team, how can the American not want to support our side. Its a perfectly natural response, because on the surface war is a giant, deadly-serious, high school football game.