I think without the US in the war the Entente doesn't have the manpower or financial capital to knock out Germany. Germany can't knock out the UK and I'd give them a 40% chance of bringing France to the table and an 80% chance of knocking out Italy (although this doesn't help much). I can see the Ottomans and Bulgaria peacing out as OTL, honestly, it'd be a close-run thing if Austria-Hungary collapses in 1919 or they make it into the 1920s. Maybe an Italian collapse keeps them afloat for now? If Germany knocks Italy out and Austria looks on the verge of collapse then perhaps we just get a kind of peace of mutual exhaustion. Germany and France can't keep going, the UK is out of capital and there are revolutions or civil wars in Italy, Russia, and the former Ottoman Empire, while Austria is on the very edge of collapse.
It's probably very unlikely but part of me really likes the idea, narratively speaking, of all the major powers calling a ceasefire as their domestic politics are swept by revolution. Germany, France, Italy, and Austria, each calling more and more troops back from the lines as insurrection and mutiny overtakes them until the trenches become slowly abandoned and everyone forgets that they're supposed to be at war. Perhaps the lack of American intervention is the only thing that could have resulted in successful leftist revolutions all over Europe (although the strength of their Marxism may vary). That'd be an interesting timeline.
It's probably very unlikely but part of me really likes the idea, narratively speaking, of all the major powers calling a ceasefire as their domestic politics are swept by revolution. Germany, France, Italy, and Austria, each calling more and more troops back from the lines as insurrection and mutiny overtakes them until the trenches become slowly abandoned and everyone forgets that they're supposed to be at war. Perhaps the lack of American intervention is the only thing that could have resulted in successful leftist revolutions all over Europe (although the strength of their Marxism may vary). That'd be an interesting timeline.