There was a time, in WWI, where the British and the German soldiers had stopped fighting for Christmas. They heard singing coming from each other's camps and started to join each other. They ate, drank, made merry, talked with each other, etc.

The next day, both armies were reluctant to start shooting at people they'd just started to befriend, and their commanders, who had not participated in this impromptu-truce, deliberately brutalized their own soldiers, forcing them to start fighting again at, literally, gunpoint.

The next year, at Christmas, any soldier that tried to have another Christmas Truce was immediately shot and killed by their commanders.

Why? Because their commanders were afraid that if the soldiers did this again, they would not return to fighting again. And that this behavior would spread to other military units, which would stop the war entirely.

A young private, at the time, in the military, Adolf Hitler, especially was pissed as hell at his fellow German soldiers for such a truce and sided with his commanders about it.