In 1914, soldiers celebrated Christmas by temporarily calling off World War I

In December 1914, Europe was locked in the bloodiest struggle it had ever known. Yet for a few hours on Christmas Day, 1914, the troops on either side of the declared a spontaneous truce. Ordinary soldiers came out of their trenches, exchanged souvenirs, and may even have played a game of soccer.

This Christmas Truce has become one of the most famous stories of World War I. It's an inspiring story of ordinary people carving a few hours of peace out of a senseless war. And for political scientists, it teaches important lessons about the nature and origins of cooperation.

By December 1914, the war seemed increasingly pointless

French troops in a ditch in 1914. These ditches would evolve into full-scale trenches. (Via Oxford University Press)

German military planners had hoped to end the war quickly by sweeping down through Belgium to conquer Paris. It didn't turn out that way. The French and British counter-attacked before the Germans reached the French capital, and the conflict soon bogged down into trench warfare.

This was a style of warfare the world had never seen before. Whereas previous wars had involved a series of discrete battles in specific locations, the Western Front stretched for hundreds of miles. Earlier battles tended to last a few days. Fighting on the Western front dragged on for months without either side making gains.

Over and over again, soldiers were ordered to throw themselves at enemy lines only to get mowed down by enemy machine gun fire. Poor sanitation and the constant roar of artillery made the living conditions in the trenches miserable. By the closing weeks of 1914, many soldiers had grown fed up with the conflict.

Spontaneous cooperation breaks out

A filmmaker plans a position for his camera in a British trench during the winter of 1914-15. (Geoffrey Malins)

The armies along the Western Front weren't very far apart; in many places they were within shouting distance. And as weeks stretched into months, the soldiers became increasingly familiar with their counterparts in the opposite trenches.

Over time, cooperation became more common. This began with efforts to retrieve bodies for proper burial. During the day, soldiers would be ordered over the top on futile charges at enemy lines. Many of these soldiers would be gunned down and their bodies would be left stranded in the no-man's land between the trenches.

After darkness fell, soldiers would venture out to retrieve their fallen comrades. And often, enemy troops would deliberately hold their fire. Over time, tacit cease-fire agreements would develop along parts of the front, with each side holding its fire while the other ventured out to retrieve dead bodies.

Every day after dusk, food would be brought up to the troops on either side of the front. And in many places the tacit cease-fire would be extended to these meal times. After crouching in their trenches for hours, soldiers on both sides appreciated the opportunity to relax while they ate dinner.

One British officer who visited the Western Front wrote that he was "astonished to observe German soldiers walking about within rifle range behind their own line," with British troops making no effort to gun them down. And these ceasefires proved remarkably durable. When fresh troops were rotated to the front lines, the departing troops would fill the newcomers in on the tacit rules that had been negotiated with the other side.

Informal ceasefires blossom into the Christmas Truce

British and German soldiers in no-man's land on Christmas Day 1914. (Via Imperial War Museum)

Sporadic ceasefires during the early months of the war laid the groundwork for the Christmas Truce. In one version of the story, the truce began when a group of German soldiers began singing "Stille Nacht," and soldiers on the British side responded with their own rendition of "Silent Night." A few troops emerged from their trenches into the no-man's land in between. When they weren't gunned down, others followed. The men reportedly sang more carols, shook hands, and exchanged gifts.

Scenes like this were reported all along the Western Front. Here's how one British soldier told the story:

About 10 o'clock this morning I was peeping over the parapet when I saw a German, waving his arms, and presently two of them got out of their trench and came towards ours.

We were just going to fire on them when we saw they had no rifles, so one of our men went to meet them and in about two minutes the ground between the two lines of trenches was swarming with men and officers of both sides, shaking hands and wishing each other a happy Christmas.

The standard story of the Christmas Truce includes a soccer game played in the space between the trenches, though historians haven't been able to verify whether this happened. The ground probably wasn't flat enough for a decent soccer game, and an organized soccer match would likely have been shut down by senior officials. But at a minimum it seems that someone produced a soccer ball and men began kicking it around.

The end of the "live and let live" system

In an influential book, the political philosopher Robert Axelrod argued that game theory can explain this spontaneous outbreak of non-combat. Axelrod's key insight was that the static nature of trench warfare — with each British or French unit occupying a position opposite a specific German unit for days or even weeks at a time — changed the incentives of people on both sides.

Senior military officials on both sides hated these tacit ceasefire agreements

Both sides knew that a vigorous attack was unlikely to achieve decisive victory. It was much more likely to produce a counter-attack that would get people on both sides killed. And because units faced the same opponents for an extended period, they knew that restraint today could encourage their counterparts to show restraint tomorrow. Over time, soldiers figured out that it made more sense to hold their fire.

Senior military officials on both sides of the conflict hated these tacit ceasefire agreements, and they were particularly appalled by the Christmas Truce. They made increasingly aggressive attempts to suppress such cooperation.

But it was hard to force rank-and-file soldiers to kill each other if they didn't want to. You can order a soldier to shoot, but it's hard to tell if he's trying to hit his targets or deliberately missing. Axelrod reports that soldiers would engage in ritualized gunfire, shooting every day at exactly the same targets. These attacks satisfied headquarters without doing serious damage to the enemy.

But eventually, the generals figured out a way to put an end to this kind of insubordination: they ordered targeted, small-scale raids against enemy positions. Units were ordered to advance until they had either taken enemy troops or suffered casualties themselves. Since units had to either produce enemy bodies or lose men themselves, these raids were difficult to fake. In the months after the Christmas Truce, the live-and-let-live system the troops had developed began to break down.

Read more

• 40 maps that explain World War I
• How World War I began, in one quick video
• When the Great War became World War I, in one chart

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