As for your POD, what about the Morocco Crisis of 1905 leading to a short war.
It wouldn't be called World War One, let's say "The Moroccan War."If Germany is defeated quickly, and Alsace-Lorrane returned to France, Wilhelm II may be itching for a rematch. Hence, they try even more of a buildup and make more of an investment, leading to the breakthrough of the Haber Process a little earlier, with more having been ivnested.
Meanwhile, Teddy Roosevelt was unable to mediate the Moroccan War and in fact a U.S. ship was sunk that caused everyone to want to gear up for war; TR, being rather jingoistic, agrees. Though it's over before US troops get overseas, this leads to a wve of isolationism and William Jennings Bryan becomes the next President in 1908. (There was also OTL, an economic downturn, and something could happen to Taft; who knows, *he* could be on the boat that sinks.)
So, Bryan is serving his first term and will be elected to a 2nd (bonus - no Wilson and Southern Democrats never start to take power with him leading them), because the US is remaining neutral despite the mess in Europe. Germany rattles its sabres more in the 2nd Morocco Crisis when events lead to the Austrian Emperor getting assassinated (a plot existed in 1911); however, there are enoguh plotters that after Germany and Austria go to war against France, Britain, and Russia early in 1912, Franz Ferdinand is also assassinated some years later.
The US can then be stalled because all Bryan wants to focus on is Free Silver, a ship that sailed over a decade ago, though he does appoint some good, liberal Justices. You can decide if TR is elected in 1916 or not; it might be Hughes because he wouldn't be on teh Supreme Court in 1910 TTL.
Okay, sorry, i know you want Europe but as an American and with you not being as much of an expert, as you stated, I thought I'd add that stuff.
Anyway, so you hve a World War One that starts, let's say, around the start of 1912. You can manipulate thigns to have it lasting longer because some sides don't get involved right away, maybe Brusilov or some other good generals for Russia aren't around, etc.. (Or, you *could* manipulate the 2nd Moroccan Crisis to occur a little later, bring the war to August of 1912 it's easier for the Germans to really get a buildup with the slightly earlier Haber process.)
The Southern Democrats, finding themselves ignored somwhat in the party, try to take power at the 1916 Convention, but Bryan backers insist on a Northern man like him. Meanwhile, the REpublicans have their own split; TR didn't try in 1912 brcause Bryan seemed unbeatable and his man had lost in '08 anyway and the GOP goes with the more conservative Knox, but now it's 1916 and TR expects to get the nomination again. So, the GOp also splits and you have a 4-way election that brings chaos and instability.
Whatever else you do, US forces could get sent over to Europe - to a war that is already at exhaustion rates - and take the Spanish Flu with them (it became most pronounced in Spain but I've read it originated in the US). it's even more devastating. Meanwhile, because Lenin, Trotsky, and Stalin can go to different places, they decide to spark revolts in Germany and France as well as Russia. The exhaustion is so enormous that pretty much each country is bled white.
Or, an alternative, the US never does get involved (as long as you keep Wilson and his cronies out I don't mind) and the Spanish Flu smply keeps cycling around the US, which maybe is weakened by Mexican rebels and the fighting is in North America. It still reaches Europe and devastates it (because Europe is alrady a mess anyway and OTL it reached China) but it's a touch worse in the US.
Does that sound good for a POD and a lead-up to what you want?