Wars are complex difficult events but when you simplify the act of fighting a war to its bare essentials the way to victory is to kill more of the enemy than the enemy can kill of your side. Ultimately, if you can kill all of the enemy you will likely succeed in the political aims of the war. These days no one can kill all of their enemies; international law requires warring parties to leave plenty of enemies alive, and in those you leave alive you leave a breeding ground for generations of enemies that will plague you with endless war.
In these circumstances it is better to be kind to your enemies and treat them well, so that they will cease being your enemies. War leaders find it hard to be kind to their enemies during a war. They have fed the people they lead with so much propaganda about the enemy, that the leaders end up believing the propaganda that they themselves commissioned. War leaders claim victory when a war is ended, but fail to understand that the military objective of a war is not to win it, but to win the peace that follows. Winning a war is relatively easy but winning the peace is immeasurably harder and more laudatory.