Christmas Miracle
This is a little farfetched, but it could've happened. If for some reason the Christamas truce kept going, with troops crossing lines, fraternizing with the enemy, I could see some of the higher ups coming in person to enforce their orders. It's concievable that seeing the carnage of the front in person, and being confronted with hordes of their own troops determined to end hostilities, could've effectively given them a dose of "Oh My GOD. What the fuck are we doing?" and from there maybe to rulers, politicians, etc. Larger media covereage of the Christmas truce could lead to popular support for the movement. Certainly in countries like France and Germany it could easily lead to active revolts, their goverments would want to agree to a peace just to deal with that, if they even managed to retain power in the face of such opposition. All in all though, in order for this to work something truly remarkable has to happen. The men at the front would have to find something to overcome the forces of nationalism that led them to the war, and that just isn't something I could see happening without some kind of major POD, probably something to redefine national identities entirely. All in all, as nice as it would be I gotta call this one ASB.