How? I doubt any country would be interested in the least if that happened. Maybe if the war got totally out of control and people were crossing the Canadian/Mexican borders, then things could escalate into something bigger. Ya some countries might get involved if that happened. But it damn sure wouldn't be because they care about the US. The US provides military support and funding to several countries. And some big countries like China would suffer terribly if the trade between them and the US was disrupted or ended in any way.
Well, EU and a lot of our allies would be pressured into picking a side by the fighting factions in the U.S
Don't you think if Texas waged war on California (Ik dumb example
) that they would be asking for Mexicos support?
Other countries would be dragged in because the U.S policy is very involved in the rest of the world. If Israel wants there financial support they better step in, same goes for pretty much all of the world.