What's also interesting is how a casus belli can evolve after a war, when the facts finally become clear. Consider America's entry into World War II. Ask a mod

What's also interesting is how a casus belli can evolve after a war, when the facts finally become clear. Consider America's entry into World War II. Ask a modern American, or anyone really, what the justification for entering the war was and they'd probably say to stop Hitler. Asked why they'd care about stopping Hitler, and the answer might be that Hitler killed several million Jews. A pretty good reason. But Americans in 1941 had no idea about the Holocaust. Yes, there had been reporting on persecution of Jews under the Nazis, and there's still controversy over who knew what, when, and how, but in general they didn't know. So why did the USA still go in? Well, no doubt there were still good reasons. But they become a lot less cut and dried when we take into account what was known at the time. The real reasons at the time were geopolitical. Only later did the moral case become obvious, and retroactively applied. I sometimes wonder what might have gone differently if the Nazis were not quite so cartoonishly and stupidly evil, confining themselves to "just" taking over Western Europe and not trying their hand at genocide quite so enthusiastically, or if Japan had not been so foolish and arrogant as to attack Pearl Harbour. Would the USA have stirred in this alternate universe, if Hitler had one less screw loose and Tojo had one fewer bottles of sake? If not, history could have easily turned out very differently.