Stahlhelm, Shrapnel

As the war entered the trench warfare phase, the number of casualties on all sides suffering from severe head wounds (often caused by shrapnel) increased dramatically. The French were the first to see a real need for more effective protection — in late 1915 they began to issue Adrian helmets to their troops. The British and Commonwealth troops followed with the Brodie helmet, which was also later worn by US forces, and the Germans with the Stahlhelm.

Source: Wikipedia > Stahlhelm