this is more clear in the context of WWI: an austrian is shot in serbia, because serbia wants independence, but russia intervenes because it wants to control serbia, and germany & austria fight russia, and france has an alliance with russia, and germany always wanted to fight france anyway, ……… the level of intervention and trying to intervene in other countries' affairs leads to a chain reaction that brings about calamity.
on the other hand, the US comes in and there's already a conflict going on elsewhere, it's not quite clear why it's happening, but it looks like it's bad, so “we can go over there and shut up the bad guys” *star spangled banner blares*