My initial thought is the attack on Pearl Harbor started the most costly and extensive war in the history of the entire world. Pearl Harbor was an attack on a military installation so it hurt the USA far more militarily, while 9/11 was directed towards civilian targets (except the Pentagon). As horrible as the effects were, Pearl harbour was a legitimate military target in wartime. The twin towers in peacetime were far less of a legitimate target to be attacked. They were innocent civilians and were not a legitimate military target as a military installation would have been. Being attacked goes with the territory if you are in the military and at least you have a chance in theory to defend yourself and your comrads but attacking innocent civilians can never be acceptable.