Ok, I'll give it a shot.
2nd International decides that all socialist parties shall refuse to vote for war credits (IOTL it fell apart as socialist parties in the nationalist fervor voted for war credits in the national parliaments).
2nd International remains in place and staunchly internationalist and pacifist.
Fritz Haber suffers an accident autumn 1914 and cannot be part of the industrialisation of the Haber-Bosch process, which means Germany runs out of gunpowder by Spring 1915. At the same time, Britain and France are running out of credit and are suffering from socialist agitation taking root in their armed forces - more in the French than in the British, but still. The Germans and Austrians suffer the same problem, but hide it better.
By summer 1915 the parties meet at the negotiaton table. Serbia and Montenegro is restored, France gets Alsace and Lorraine. The Ottomans gain Kuwait and the Germans have to trade away Togo and German Southwest Africa.
Itlay remains out, USA is isolationist, Russia is shaken by revolution along the lines of 1905, but goes constitutional monarchy instead.
With the Balkans dominated by Austria, which developes economically and becomes more of a federation under Emperor Karl and Eastern Europe by Russia and Germany, you eliminate many nasty little "I want to kill my neighbour" by making them police actions against small disenfranchised terrorist cells rather than causes of war and genocide. Likewise, with a strong and reforming Ottoman Empire. muslims have something to work with rather than turn all thier hatred towards colonising powers.
A hundred years of peace after this, perhaps?