World War I Christmas Truce.

This truce was not authorized by the commanders of either country (Germany and Great Britain). When soldiers on both sides started getting out of their trenches and making friends with their enemies, the commanding officers on both sides didn't know what to do. Some soldiers got out of the trenches and befriended the enemy, some didn't. The ones that did got in trouble later.
When they all finally went back to their own trenches and ended the truce, a lot of the soldiers were unwilling to fight against the enemy,   because they had made friends with each other.
High ranking officers of both sides told the soldiers on their side not to participate in the truce. If they got out of their trench, they'd be in trouble. So many soldiers disobeyed the direct order that there wasn't much the commanding officers could do.
Since there were so many soldiers on both sides that were now unwilling to fight against their new friends on the other side, a lot of soldiers on both sides had to be relocated to different locations far away. That way they faced different enemy soldiers whom they did not know and they had not made friends with.
When WWI was over, The militaries on both sides wrote some new military regulations that made it illegal for any of their military personnel to do something like this ever again. They also came up with some very harsh penalties for anyone who ever participated in a truce like this again. The most severe penalties included: Court Marshall and military discharge. Soon after these countries made rules against having a truce like this, other countries made their own similar rules against it. For the rest of the war, some unit commanders on both sides ordered artillery strikes on the other teams trenches on Christmas eve and Christmas day, just to prevent anything like this from ever happening again.
(There was a documentary on the History Channel about this several years ago.)

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