THE SPARK: Honestly? I think the U.S. will initiate it. Society here has changed since the last World War. We're all up in people's business, more and more treating the issue of other countries as our own. Unfortunately, that's like a policeman out of jurisdiction without the benefit of hot pursuit rules. Eventually, somebody takes precident to this. And because the U.S. is a big place to fuss with, ALOT of somebodies decide to join in. Not like Red Dawn, but a number of countries who - of their own volition and spacing themselves from other factions - want the U.S. up out of their grill. Maybe alliances will be born, but notably no super powered nations will be involved, citing this as perhaps a bad idea.
WHEN: Probably the next time U.S.A. does another high-profile witch hunt on a nation or forces their unwanted hand heavily into another nation's territory with another round of going too far. Now understand...I live in this country. Nobody can appreciate better on who's going too far than the side with the biggest guns to point. Picking on little nations who aren't even Nazi-level aggressive is embarassing. Either the U.S. needs to find bigger boys to play with or become the villain. So, we cause enough unrest in a country and THAT country says "What the fuck?" and decides to get their game on. Other nations then follow suit by either agreeing or having their own grudge on the table.
WHAT WILL HAPPEN: Again, this isn't Red Dawn. I said that significant super powers will keep out of this fight unless it's China and the U.S. calls them. I'll get to that later. Basically, smaller nations do their thing, whatever they can think of. Invasion forces, guerilla attacks, etc. This causes alot of chaos and problems, forcing U.S.A. to put itself in real-time war status, committed on its own shores. It's a problem, because alot of smaller dogs going after a big dog is frightening, confusing, and it hurts. Maybe this puts a strain on resources, because the nation isn't at its fullest to defend its borders right now.
What happens then? Well, they do what I figured they do... They call for aid from China. Why China? Because they're big, they're bad, and they have NUMBERS. Lots of soldiers or potential soldiers to buff the ranks. They have to call in a treaty of understanding first over the matter of the U.S. oweing them money and the U.S. saying "You don't get a cent until you get rid of these fucking stink bugs.", but they work something out because the U.S. DOES owe China something, and that puts them at relative interest. Namely, if the guy who owes them money dies, THEY DON'T GET PAID. So, at the risk of a bigger debt, America outsources to China yet again and they get more soldiers.
Who Wins?: Probably the U.S. Even without China or some other helping nation, we're talking about smaller nations with smaller resources overall. The big ones don't want to fight because there comes a time when WE ALL LOSE and nobody wants that. So, the U.S. gets punched in the face a few times and gets the bad guy badge until it makes nice again. They probably don't lash out at the home nations of the attackers because - yeah - pushing our hands out all the time like this clearly causes problems. A lesson is learned, we hope. Because if we can't figure this out on our own, someone will have to force the issue in answer to some point where we go to far, and hopefully the bloody conflict will enlighten us.