Powerful Impetus

In the winter of 1914 nothing seemed more unlikely than soldiers venturing across no-man's land to lay aside their arms and mingle with the enemy. This was the famous Christmas truce of World War I.

Stanford Professor Robert Sapolsky analyzes how such cease-fires can develop while war is raging.

Preadaptation: There tended to be a lull in the fighting during meals. Those pauses existed for the simple reason that no one, on either side, wanted to interrupt dinner to kill or be killed. But these lulls began to be used as ways to send signals to the other side....So the soldiers would make a point not just of shooting less frequently during dinner: They would let the guns thunder until the stroke of 6 p.m. and then go utterly silent until 7 p.m., every day. And if the other side started doing the same, they had essentially negotiated a narrow truce: no fighting during dinner.

Problems: cheating, when one side takes advantage of a truce to attack, and signal errors, mistakes made when, say, some clueless newbie lets loose with a shell. [snip]

the best way to police against cheating and signal errors is a variant of tit-for-tat—that is, retaliation that is not much more than the magnitude of the violation—and then a return to cooperation....the typical response to a violation of “live and let live” would be retaliation of roughly twice the magnitude.

In the early stages of the war, before the carnage rendered the enemy "sub-human",

Such truces emerged repeatedly during World War I, and just as often, the brass in the rear would intervene by rotating troops, threatening courts-martial and ordering savage raids requiring hand-to-hand combat—all to shatter any sense of shared interests between enemies. And still the truces would start up again.

The motivation not to be killed oneself is a powerful impetus to peace.