America's involvement in World War I brought new names for combat stress reactions , such as "shell shock," which more accurately reflected what was actually causing most of the combat stress trauma. World War I was a war of trench warfare, where thousands of miles of trenches were dug when fighting this war.. Soldiers were shoulder to shoulder in these trenches and when advancing out of their trenches, were cut down by machine gun fire and barrages of artillery shelling. Therefore it is not surprising due to the battlefield strategies employed, approximately seventy-five percent of all casualties in WWI were caused by artillery shells. Additional casualties were the result of poisen gas, which was a first for any war up until this time.
