A soldiers worst nightmare
Probably the most feared weapon during WWI was being attacked by poison gas. Unlike incoming artillery or machine gun fire which would be generally quick and painless, poison gas would usually kill you slowly and effects could last over a few days. The Germans waited for a favorable wind and opened up tear gas cans which forced opposing troops to remove their gas masks. Following that they would release mustard gas, a much more toxic gas which attacked the respiratory system, a gas mask offered very little protection to this attack. By the end of the War the casualty count of the gas attack reached over 1 million.