I suspect most of you will say WWII as it had greater causalties, especially civilian causalties, and Im sure some will bring up the atomic bombings and fire bombings which are fair arguments. But I'm going to have to go with The Great War WWI. Simple. It was so damn pointless. The war started for practically no reason other than countries not liking each other. There was no clear goal for either side during the fighting, and absolutely nothing was accomplished other than laying the ground work for another War. WI's battles were absolutely awful as well. Essentially rot in a trench hoping to not get gassed or hit by artillery and wait for your CO to blow the whistle. When he did....you die. No tactics at all, just charge straight into machine gun fire. The Battle of the Somme was so brutal that the ground was more blood and bodies than it was earth.
Thanks I didn't find the right solution from the Internet.