Germany and Japan in 1945 weren’t run by savages who hated each other’s sects of the same religion.
You can't be serious. Have you heard of the Holocaust? Or the rape of Nanking? The Germans and the Japanese committed atrocities on a much greater scale than ISIS has. Both Germany and Japan were filled with world class savages back in the day.
And how is that different from Communists ready, willing, and able, to pick up the pieces if we didn't?
Sorry. I forgot to add I think your distinction is arbitrary (obviously, the presence of Communists, and the existence of warring sects are different).
My point is the dynamics of a fallen or conquered government are exactly the same.