Plainly people are rational (and in a more arguable and asterisked way rational in the sense of results-maximizing, be it for self, in-group, whichever) a lot of the time. There's no question of rejecting the idea of rational decision-making and interest-weighing in politics. It's just the exceptions to that rule - people acting out on the basis of irrational considerations like identity feelings or personal or group honour, or moral sentiment - are better considered as genuine exceptions than as something that can be integrated into an (excessively) broad rationality model. For that one need not even look to the historical record (although it'd certainly do the job) but merely one's own life experiences. Often, people are rational, often enough for it to be a basic framework for understanding behavior. Quite often, they aren't. Think of the most irrational things you and others have done - best understood as some sort of topsy-turvy "special rationality on the basis of unusual information," or just as irrationality, most easily understood on its own terms (sentiment, etc.)?