I still hate WW1. It seemed stupidly obvious almost immediately in WW1 that the old tactics of charging across open terrain in mass had become outdated overnight due to technology like machine guns and better artillery that now wiped out whole units at once. And yet they continually did basically the same thing until 9,000,000 soldiers were dead, and usually for little or no tangible gain of territory.
Compared to other wars where brilliant, unorthodox and daring new strategies were used to win legendary battles that are still studied today like Cannae and Trafalgar, someone should've realized that even the newest inventions like tanks and warplanes weren't winning decisive victories and stopped sending guys out of the trench to slaughter until they could figure something better out. The tactics used really never caught up to what was going on and it resulted in an uncountable number of needless casualties. And if that wasn't enough of a failure the resolution to WW1 was a catastrophe and lit the fuse for WW2 along with much of the conflict still going on in the Middle East.