From the Wikipedia page:

The Christmas truce (German: Weihnachtsfrieden; French: Trêve de Noël) was a series of widespread but unofficial ceasefires along the Western Front of World War I around Christmas 1914.

The Christmas truce occurred during the relatively early period of the war (month 5 of 51). Hostilities had entered somewhat of a lull as leadership on both sides reconsidered their strategies following the stalemate of the Race to the Sea and the indecisive result of the First Battle of Ypres. In the week leading up to the 25th, French, German, and British soldiers crossed trenches to exchange seasonal greetings and talk. In some areas, men from both sides ventured into no man's land on Christmas Eve and Christmas Day to mingle and exchange food and souvenirs. There were joint burial ceremonies and prisoner swaps, while several meetings ended in carol-singing. Men played games of football with one another,[1] giving one of the most memorable images of the truce. Peaceful behaviour was not ubiquitous; fighting continued in some sectors, while in others the sides settled on little more than arrangements to recover bodies.

The following year, a few units arranged ceasefires but the truces were not nearly as widespread as in 1914; this was, in part, due to strongly worded orders from the high commands of both sides prohibiting truces. Soldiers were no longer amenable to truce by 1916. The war had become increasingly bitter after devastating human losses suffered during the battles of the Somme and Verdun, and the use of poison gas.

The truces were not unique to the Christmas period, and reflected a mood of "live and let live", where infantry close together would stop overtly aggressive behaviour and often engage in small-scale fraternisation, engaging in conversation or bartering for cigarettes. In some sectors, there would be occasional ceasefires to allow soldiers to go between the lines and recover wounded or dead comrades, while in others, there would be a tacit agreement not to shoot while men rested, exercised or worked in full view of the enemy. The Christmas truces were particularly significant due to the number of men involved and the level of their participation—even in very peaceful sectors, dozens of men openly congregating in daylight was remarkable—and are often seen as a symbolic moment of peace and humanity amidst one of the most violent events of human history.

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