The correct answer to this open question is the following.
The warfare that most American serviceman engaged in was Trench warfare.
During World War 1, Trench warfare was used when the soldiers occupied fighting lines and protected themselves from the shooting attack of the enemy. In World War 1, Trench war in the West front generated attrition for both armies, the Entente Alliance formed by Russia, France, and Britain, and the Central Powers, formed by Germany, Austria-Hungary, and the Ottoman Empire. In this type of warfare, the defensive position had the advantage because it was trenched, just repealing the attack of the enemy that was interested in taking control of the position.
