The poison grenade was probably the most deadly weapon in the war. Especially because they were fighting in trenches. It killed and hospitalized tons of soldiers. Especially because finding a gas mask was very rare.
The next most deadly weapon is the machine gun. It was able to take out many soldiers at once. Especially when the men ran out into no mans land they were usually killed because of all of the bullets flying everywhere.
Next is the flamethrower, it does not have a lot of distance but can take out lots of men at once up close. It can burn down buildings, and do many other deadly things.
Next are the mines, they are hidden in the ground and explode when stepped on. They explode very fast when stepped on and can take out lots of men at once if near.
Next are the barbwire fences. They can prevent soldiers from moving in. They can also be a distraction for the soldiers, which makes any easy shot.
Next is the tank, it is massive and has lots of armor. It fires giant bullets and explodes on impact. Although since it is so big many heavy weapons can take it out.
Next is the artillery, it is very useful for taking out tanks. It can take out ground troops as well. It has lots of armor as well as the tank.
Next are the mortars. They are basically giant exploding cannon balls. They can be shot from a very far distance away. It is very deadly.
Next is the air craft. This can drop bombs and shoot many bullets at once. It can take out other aircraft's as well. It was very useful in the war.
Next are the submarines, they help destroy enemy ships stealthily and fast.