Yes, unfortunately.
Deliberate targeting of civilians, especially with the overriding goal of terrorizing the survivors, is contrary to the principles of a just war.
Thomas Aquinas and Augustine were the originators of the Just War theology, and had great influence in its development. Even in the height of the influence of the theology, it was considered just to totally destroy and enemy city after a siege. The reason? That would convince the other enemy cities to surrender and there by save more lives. The more merciful commanders limited it to a three day sack. This was in the age of Chivalry, during the times where many leaders really did try to limit the fighting to smaller engagements. It was also when an army on campaign was considered just in seizing any materials along the way to support itself.
The idea of Just War is not quite what is often presented. The point was to limit the death and destruction to a minimum. The atomic bombs, for all their horror, did that.