The Most Powerful Weapon Of WWI: Poison GasJade Adams
Poison gas quickly and easily burned and blistered people and ruined lungs and other internal parts of anyone it encountered causing slow painful deaths.
It's a much easier way to kill off large groups of people without turning it into a huge bloody mess. The few issues they had with gas were resolved too so there's mostly no flaws within it.
Even the soldiers that somehow survived the gas were usually unable to continue to fight because the injuries were usually extreme. (Shown below)