From Smithsonian Magazine:
And then the war continued . . .
The first signs that something strange was happening occurred on Christmas Eve. At 8:30 p.m. an officer of the Royal Irish Rifles reported to headquarters: “Germans have illuminated their trenches, are singing songs and wishing us a Happy Xmas. Compliments are being exchanged but am nevertheless taking all military precautions.” Further along the line, the two sides serenaded each other with carols—the German “Silent Night” being met with a British chorus of “The First Noel“—and scouts met, cautiously, in no man’s land, the shell-blasted waste between the trenches. The war diary of the Scots Guards records that a certain Private Murker “met a German Patrol and was given a glass of whisky and some cigars, and a message was sent back saying that if we didn’t fire at them, they would not fire at us.”
The same basic understanding seems to have sprung up spontaneously at other spots. For another British soldier, Private Frederick Heath, the truce began late that same night when “all down our line of trenches there came to our ears a greeting unique in war: ‘English soldier, English soldier, a merry Christmas, a merry Christmas!’” Then–as Heath wrote in a letter home–the voices added:
‘Come out, English soldier; come out here to us.’ For some little time we were cautious, and did not even answer. Officers, fearing treachery, ordered the men to be silent. But up and down our line one heard the men answering that Christmas greeting from the enemy. How could we resist wishing each other a Merry Christmas, even though we might be at each other’s throats immediately afterwards? So we kept up a running conversation with the Germans, all the while our hands ready on our rifles. Blood and peace, enmity and fraternity—war’s most amazing paradox. The night wore on to dawn—a night made easier by songs from the German trenches, the pipings of piccolos and from our broad lines laughter and Christmas carols. Not a shot was fired.
And then the war continued . . .
