Versailles Treaty, specifically the harsh terms and blaming Germany for the war. (It was actually the least to blame)
Whatever extremely smart & sadistic/idiotic and delusional person/group created the Torah and the New Testament. (I have no true quarrel with Hinduism, and Muslims are in my view a whole lot less violent, and I'm atheist)
Operation Market Garden
Allied over-cautiousness in WW2, could've shortened the war by months if they had some backbone and took a chance whenever there were huge opportunities.[/QUOTE]