With British and German forces separated only by a no-man's land littered with fallen comrades, sounds of a German Christmas carol suddenly drifted across the frigid air: "Stille Nacht, Heilige Nacht" ("Silent Night, Holy Night").

Then, during that first Christmas Day in World War I, something magical happened.

Soldiers who had been killing each other by the tens of thousands for months climbed out of their soggy, muddy trenches to seek a shred of humanity amid the horrors of war.

Hands reached out across a divide the narrow divide, presents were exchanged, and in Flanders Fields a century ago, a spontaneous Christmas truce briefly lifted the human spirit.

"Not a shot was fired," Lt. Kurt Zehmisch of the 134th Saxony regiment wrote with amazement in his diary that Christmas.

On the other side of the front line, Pvt. Henry Williamson of the London Rifle Brigade was amazed by the goodwill among his enemies. "Yes, all day Xmas Day & as I write. Marvelous, isn't it?"

Few could be believe their eyes, especially on this mud-caked patch of Belgium and northern France where crimson poppies had long ago shriveled in the cold.

Peace allowed for corpses to be recovered from the fields and given a proper burial. Fighting continued in many other places on the front line. But it was a momentary peace in a war that would last for nearly four more years.

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THE BIRDCAGE

Near one of the spots where British and German soldiers fraternized for the unexpected truce, a dark, dirt track veers off the road and meanders into the gloom of the woods.

There, a cleared space has the graves of British soldiers who died on Dec. 19, 1914, in a battle as gruesome as it was insignificant, their dreams of a peaceful Christmas ignored and buried in the cold mud.

It was a time when swift military movement from Germany across France to the Belgian coast was grinding to a stalemate, leaving hundreds of thousands of casualties behind. For both sides — Germany versus an alliance led by France and Britain — this buried any hope that the war would be over by Christmas.

The result was a form of warfare across trenches where human life was expendable.

"There are a number of local attacks which never make it into the history books, but which all cause a great loss among the troops," said Piet Chielens, curator at the In Flanders' Fields Museum in Ypres, Belgium.

The Dec. 19, 1914 "Birdcage" attack occurred on a bulge of the German line about the size of a football (soccer) field. Allied soldiers also had been thinking about Christmas, but for 80 of them it turned into disaster in an area where a warren of barbed wire had given the German defenders a huge advantage.

Chielens said that during those early days of the war both sides dug into the Flemish soil, with commanders "attacking without deep thought, without deep concern about the fate of their men." The infamous "Birdcage" was one of those battles that made them realize that strategy wouldn't work.

With offensive artillery nearly non-existent and sometimes so wayward that it also was a threat to its own troops, soldiers were thrown into scenes where one machine gun could mow down a whole row of approaching men.

Some of the bodies found after the attack were so mangled they could no longer be told apart, and today the headstones of several casualties stand shoulder to shoulder to mark that horror.

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"WE NO SHOOT"

Little wonder so many soldiers were pining for a glimmer of hope on Christmas Eve.

Frank and Maurice Wray of the London Rifle Brigade settled in to keep watch when they suddenly heard a German band in the trenches play songs "common to both nations," they later wrote in an article. "Quite understandably a wave of nostalgia passed over us."

At dawn, a German called out, "We good. We no shoot," and the Wrays noted: "And so was born an unofficial armistice." Men walked out, extremely apprehensive at first, many fearing some deadly trick. Then human warmth cracked the freezing cold.

Chielens said that at about 30 scattered points across many miles (kilometers) of Belgium, similar scenes occurred. Others happened across the Western Front, which ran from the North Sea to the Swiss border.

Apart from talk in a shared language or merely with hands and kindred eyes, the men exchanged gifts, using everything from bully beef and barrels of beer to small mementos. Some played football.

German soldier Werner Keil scribbled his name on a piece of paper and gave a uniform button to 19-year-old British Cpl. Eric Rowden of the Queen's Westminster Rifles on Christmas Day 1914. "We laughed and joked together, having forgotten war altogether," Rowden wrote.

War, though, was never far away, including memories of the Birdcage still lingered and the burials of dead soldiers. It was hardly as if gunfire stopped all over to make room for warm embraces.

On the Belgian stretch around Ploegsteert alone, "we still have over 250 people dying on Christmas Day itself. There was enough fighting going on," Chielens said.

Despite the uplifting moment, commanders away from the trenches abhorred the softening of posture and fighting spirit. Cpt. Robert Hamilton of Britain's 1st battalion Royal Warwickshire noted: "I am told the general and staff are furious — but powerless to stop it."

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25TH DECEMBER 1917

All that would change soon enough, and the 1914 truce would not be repeated.

Once British Gen. Sir Horace Smith-Dorrien had heard of it on Dec. 27, 1914, he wrote in a confidential memorandum that "this is only illustrative of the apathetic state we are gradually sinking into." He threatened disciplinary action to avoid a repeat.

"It was certainly remembered by the army commands because around Christmas in 1915, 1916, 1917, they see to it that there is no such thing as a truce possible because then the shelling will be deliberate and very intensive."

One thing did continue — the relentless killing.

Next to a monument in Ploegsteert to mark football playing at Christmas is the Prowse Point Military cemetery. Amid the 225 dead, three headstones from the 27th Battalion Australian infantry stand out.

Date of death? "25th December 1917."

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Virginia Mayo contributed to this article.

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Raf Casert can be followed on Twitter at http://www.twitter.com/rcasert