A weapon that helped lose a war:

World War 2
Italy
Breda Safat Machine gun

The machine guns inadequacies put Italian aircraft, infantry, and tanks at a serious firepower disadvantage. The gun was the primary arm for the Italians and was inferior in almost every capacity.

The end result is Italian infantry were at a firepower disadvantage in nearly any engagement and superb aircraft were left without the offensive capability to capitalize on any of their advantages, particularly the MC 202.

The Italians were never able to solve these problems. The SAFAT served throughout the war and beyond with a modest rate of fire, low muzzle velocity, terrible ammunition, and modest to poor levels of reliability.

You could make a solid argument that this weapon, and it's many inadequacies, ensured Italian defeat.