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NuclearReactor
I don’t know exactly but i would say the machine gun maybe?

I don’t know much about the weaponry of WW1 but the machine gun along the bolt action rifle seed to have played a part in it.

The machine gun with its high rate of fier could kill a large number of people on an open field.

I am really sorry if the answer is incorrect i tried my best to help you
It was the Machine Gun as it made advancing impossible for both sides.and just led to mass slaughter and a stalemate.