The underlying sentiment may not be expressed in such a crude manner by the vast majority of people, but I think you'll find that competitive sports are based around winning, and forgive me but I think you'll find that amounts to be being better. My argument is that this is far more damaging when we're talking about competing countries - it has the potential to take on a political significance such as the case with the Nazis during the olympics. Ethnic rivalries such as those beween Celtic and Rangers is another case in hand, and I also understand this occurred among teams in the former yugoslavia.
Yes, politicians manipulate nationalistic sentiments among the populations they govern or at least have influence over, but I think you'll find it doesn't take much to inspire xenophobia. I mean take for example violent racism this has occurred throughout history, it's not always just down to politicians telling people how to think or what to do - people can think and make decisions for themselves.
Oh btw when YOU put an argument together maybe you should consider trying to exercise a little more civility, and possibly not resort to simplistic maxims like; "Politicians start wars, not tribalism", to explain away what are extremely complicated and serious issues - sorry but it's your attitude that pisses me off not your argument.