The words drifted across the frozen battlefield: ‘Stille Nacht. Heilige Nacht. Alles Schlaft, einsam wacht’. To the ears of the British troops peering over their trench, the lyrics may have been unfamiliar but the haunting tune was unmistakable. After the last note a lone German infantryman appeared holding a small tree glowing with light. ‘Merry Christmas. We not shoot, you not shoot.’ It was just after dawn on a bitingly cold Christmas Day in 1914, 90 years ago on Saturday, and one of the most extraordinary incidents of the Great War was about to unfold. Weary men climbed hesitantly at first out of trenches and stumbled into no man’s land. They shook hands, sang carols, lit each other’s cigarettes, swapped tunic buttons and addresses and, most famously, played football, kicking around empty bully-beef cans and using their caps or steel helmets as goalposts. The unauthorised Christmas truce spread across much of the 500-mile Western Front where more than a million men were encamped.

Near as anyone can seem to tell, there’s only one man alive who was in those trenches 90 years ago; the Observer has the rest of the story.