The tremendous amount of artillery fire during World War I led to many battlefield innovations. The biggest change was the trench warfare that came to define WWI. Another lasting change was the advent of metal helmets. In 1914, when soldiers first entered combat they continued to use the leather helmets of their forefathers. After countless deaths from head injuries caused by shrapnel, the French switched to metal helmets. The other belligerents quickly followed suit, and the metal helmet has been with us ever since.