Especially as British strategy for any potential conflict would be "hope Canada can hold them back long enough so that the British Navy could try to strangle the US economy and inflict enough decisive defeats that they no longer wish to maintain war." The British knew by that point there was no way they could hold any part of North America if the war dragged on long enough.
That, and the most important thing for the European empires were their ability to maintain their position in Europe. Germany may have had a few colonies in East Asia, but the US merely threatening them would not be due cause by itself to declare war. They'd be a mostly belligerent neutral, at least at the start.
For the sake of the argument, let's assume that the various empires survive until this point in time. We have, among the major powers, Great Britain, France, Germany, Italy, Austria-Hungary, Russia, the Ottomans, Japan, China, and the US. Assuming that the war begins over access to China, the US and Japan will begin with the hostilities. Assuming continued Japanese forays into Manchuria, China will focus her own actions against Japan. It's doubtful that there will be any alliance between the US and China, at least in the early beginning. They will probably be cobelligerents, more than anything.
Russia, concerned that the Japanese might take this chance to formally acquire all of Manchuria, begins their own forays against the Japanese. They are still keen to avenge their earlier loss, and the current Tsar might try to use it as a rallying cry for Russia. (...might. Details always are picky.) The separate wars continue for a little while, with a more formalized alliance being set later in the war.
Now, turn towards Europe. Russia is more vulnerable in the west with forces shifted East, and old enemies realize now might be the time to strike. The Ottomans approach any interested nation that would join in an offensive against Russia. Germany would probably leap at the chance, but Austria-Hungary might want to sit it out. They (should) know that they could hardly absorb anymore land and foreigners, and that most of the gains would be made by Germany. That, and any action in the East would make them vulnerable to Italy. Austria, here, is our first likely neutral.
Confident enough about their chances, Germany and the Ottomans (and, again, maybe Austria. Maybe.) launch their attack into Russia. Persia might join as well, as I said earlier. Perhaps even Sweden joins in. (not likely, but again, who knows? Just throwing out ideas.) That puts Russia from a good position to a really poor one, and has to rapidly try to shift forces west, draining their contribution against the Japanese in the East. It is a difficult thing to do, but, eventually, the US and China do agree to support Russia under an earlier deal, and the two sides are thus drawn.
US / Russia / China & Friends
Japan / Germany / Ottomans & Friends
Then we get to the interesting portion. France wouldn't be worried about a war half a world away, but they would be worried about a Germany with a potential to own half of Europe, even if only in theory. And, as such, they almost immediately agree to join Russia's side and assist them from the west. The revanchist sentiment may have dwindled by now, but France definitely doesn't want a strong Germany astride central Europe.
US / Russia / China / France & Friends
Japan / Germany / Ottomans & Friends
This puts Italy in an awkward state, as both sides would be interested in its assistance, but Germany makes a case for Italy getting Corsica and French colonies. At the same time, they know Germany wants Austria-Hungary to join. If they do, Italy would probably rather get Trieste, Dalmatia, and other parts of that empire. So they probably sit out for now and glare at Austria, waiting for the other to make a move.
Then, and only then, with the battle in Europe looking (mostly) even, and France stuck in the trenches. They might get this clever idea to swing around Belgium, whether or not the small country likes it. Then, and only then, might Great Britain join with Germany and the others to fight the US, as it feels that it's being pushed into the corner. So, at the very end, you get:
Team US / Russia / China / France & Friends
Team Japan / Germany / Ottomans / Great Britain & Friends
Hostile Neutral Funtime Austria-Hungary / Italy
That should get you a nice WW1.5 set up, assuming everything else is similar. It might even be relatively even, considering the US wouldn't have quite hit full stride as it did in WW2 and China can hardly send many troops west (if they even would). It would definitely make for an interesting timeline, with a reluctant Great Britain finding itself forced into a corner and forced to make an unhappy choice, either way.