At Flanders Fields in Belgium one hundred years ago in December of 1914, German troops were bogged down in trenches as close as sixty yards to British forces (along with some French and Belgian) in their own trenches. The fighting was fierce. Pope Benedict XV proposed a truce for Christmas day, but leaders from both sides roundly rejected the idea as absolutely impossible.
On Christmas Eve, the Germans started erecting miniature trees on the top of the trenches and singing carols. The British initially fired on some of the trees, but then held back to see what happened next. The Germans then propped up handwritten signs along the order of "YOU DON'T SHOOT; WE DON'T SHOOT." Eventually, both sides crawled out of their trenches, exchanged gifts and meals, buried their dead, and played soccer.
The German and British brass were incensed by this conduct and ordered their men to start fighting again. The soldiers did so in a desultory, token manner by occasionally firing rifle rounds into the air. Finally, the officers started threatening court-martials with severe penalties if the carnage did not resume, and, right around New Year's (and unfortunately, perhaps earlier in some parts of the lines), the combatants reluctantly shook hands and returned to their respective trenches to "sing a slaying song tonight" and get back to their killing.
Attempts to repeat the Christmas Truce in the subsequent years of the war failed miserably.
For an extraordinarily well-researched and thorough examination of the Christmas Truce, go to the Operation Plum Puddings website.