For this story to make sense, you have to understand the differences between World Wars I and II.
World War II was a noble war, a war that inflamed the passions of the home front and the soldiery of all the Allied countries. France was conquered; England was buckling under Hitler's constant assaults; and the United States had to fight two wars, both of them "just:" They virtually alone had to fight the Japanese in the Pacific (sure, Britain helped, but mostly just enough to get their asses kicked by the Japanese) as well as quarterback the war against the Nazis. Able-bodied men lined up at the enlistment stations by the tens of thousands to fight the "Japs" or the "Nazzees", as the vernacular of the day had them.
But World War I, or The Great War as it was called at the time, was fought because of paper.
The story starts with the Austro-Hungarian Empire, which not only comprised the two eponymous countries but also both sides of the former Czechoslovakia, Herzegovina, Yugoslavia, Croatia, Slovenia, and big chunks of other central-European countries like Romania. It was a giant empire and at the time - 1899 - one of the world's eminent powers.
The Crown Prince of the Empire, a cat named Rudolph, committed suicide, leaving his cousin Karl Ludwig as the heir apparent. Rather than take on his responsibilities as Next In Line, he decided rather to die of typhus - everyone makes choices, I guess. That left his son, the then-36 year old Franz Ferdinand, as the Royal Dauphin. He was given the title of Archduke and groomed for the eventual throne.
He spent 15 presumably happy years as Crown Prince, did Franz Ferdinand; despite his increased responsibilities he continued an aggressive schedule of leisure pursuits such as hunting and traveling, and waiting for the sitting Emperor, Franz Joseph, with whom he did not get along, to die.
However his happy life was put to an abrupt and ugly end on June 28, 1914, when an anarchist named Gavrilo Princip, who was a member of a group called Young Bosnia, shot the Archduke in the neck; he died a few minutes later. Princip was only 19. Being underage he did not receive the death penalty but was given a 20-year sentence; he served less than four years of it before succumbing to tuberculosis and malnutrition in prison. Archduke Franz Ferdinand was 50.
At Princip's trial, it came out the he had received help from a number of members of the Serbian military. This not only incensed the Austro-Hungarians but also gave them an opportunity to stir up conflict with the Serbian Empire, whom they viewed as a threat. They wrote up a series of corrective and punitive actions that Serbia would have to accept called The July Ultimatum, most of which were nothing more than blatant attempts at humiliation that the Serbians would never accept. Winston Churchill called the document "...the most insolent document of its kind ever devised." When they did in fact reject the Ultimatums outright, Austro-Hungary declared war on Serbia.
Now Serbia had a treaty with Russia, and knew that she would ally herself with Serbia. After a month of posturing and dirty politics, it shook down that Russia, Great Britain, and France were allied against the Austro-Hungarians and Germany, each nation declaring War against the other side starting July 31, 1914. The Great War had begun.
Part 2: The Fighting and the Truce
So now you had, on the war's northern front, the French and the British fighting the Germans, none of whose soldiers were passionate about the cause. In fact the Allies found that they had much more in common socially with the Germans as opposed to the more Eastern-European peoples like the Serbs, with whom they were nominally allied. Still, soldiers they were, and they dug giant trenches and fired at each other, the front lines moving inch by inch back and forth.
The common thinking among the soldiery was that the war would be coming to a swift end, being fought as it was over "a scrap of paper" - the treaties that bound one country's fortunes to another. So they were dispassionate about the cause as well as ambivalent towards the enemy himself.
Even before Christmastime, the war was remarkably civilised according to today's standards; they often agreed to cease-fire at certain times to recover the bodies of the dead. There was also an unspoken agreement between the two sides that if soldiers were outside the trenches working or exercising that they would not be fired upon.
About a week before Christmas the British forces would start hearing Christmas carols being sung by the Germans. Before long the British were singing their carols back to them.
On Christmas Eve, the German side, in addition to singing its carols, decorated their trenches with candles and any other such cheer as they could contrive. They also started shouting Christmas greetings to the British (and to a lesser extent the French), who would shout them back.
Then, remarkably, the soldiers of both sides started walking, tentatively at first, towards no-man's land. They shook hands, exchanged greetings and small gifts. Along the entire front, which stretched from the North Sea all the way down to Switzerland, over 100,000 troops laid down their arms and observed Christmas with their nominal enemies. In several areas someone would produce a soccer ball and the two sides would play a spirited game together.
Bruce Bairnsfather, a military cartoonist, remembers things this way:
"I wouldn't have missed that unique and weird Christmas Day for anything. ... I spotted a German officer, some sort of lieutenant I should think, and being a bit of a collector, I intimated to him that I had taken a fancy to some of his buttons. ... I brought out my wire clippers and, with a few deft snips, removed a couple of his buttons and put them in my pocket. I then gave him two of mine in exchange. ... The last I saw was one of my machine gunners, who was a bit of an amateur hairdresser in civil life, cutting the unnaturally long hair of a docile [German], who was patiently kneeling on the ground whilst the automatic clippers crept up the back of his neck."The two sides would often observe joint burial ceremonies, after which neither side would feel much like creating more carnage.
So Christmas Day turned into Christmas Night, and with the exchanging of gifts and pleasantries exchanged, the kick-arounds of the soccer ball ended due to darkness, and the dead buried with honor, both sides returned to their trenches, but in many places neither side chose to take up arms just yet. In many places the truce resulted in dramatically reduced hostilities until the New Year. There are several tales of each side deliberately firing over the heads of the enemy. One story, perhaps apocryphal, tells of a British soldier accidentally firing low and killing a German soldier. The Brit shouted his apologies to the opposing trench for the unintended death.
Well, eventually the Generals finally got a hold of their troops again and demanded that the soldiers do their job, which they eventually did. One soldier's account of the resumption of the hostilities had his commanding officer firing three times into the air, hoisting a flag that said "Merry Christmas," and rising out of the trench to salute his counterpart in the German trench. When they got back into the trench, the German commander fired into the air, and the truce was over.
The war did not end quickly. In fact it was 1918 before the US entered the war and tipped the scales to the Allies' side. The eventual use of poison gas by both sides would dehumanize the enemy, putting to an end any subsequent attempts at truce. But for one day, both sides in a horrible conflict put down their guns and extended a gesture of peace and goodwill to the other, and created a small oasis of humanity in the ocean of war that Europe had become. For those who took part, it created an indelible impression that they kept with them for the rest of their lives.