To drill into a point of my quoted assertion: "You don't go to war without support from your trading partners." A world war is an exceptional case. I meant to point at national conflicts, specifically between equals. I think having to defend your ally is a separate problem.
Well, you're less likely to go to war with your trading partners.
I'd have thought, at least historically, wars were more likely with trading partners - they're going to be close and, by definition, they have stuff that you want.