Trench warfare
Trench warfare prevailed on the Western Front from late 1914 until the Germans launched their Spring Offensive on 21 March 1918. After the buildup of forces in 1915, the Western Front became a stalemated struggle between equals, to be decided by attrition. Frontal assaults, and their associated casualties, became inevitable because the continuous trench lines had no open flanks. Casualties of the defenders matched those of the attackers, as vast reserves were expended in costly counter-attacks or exposed to the attacker's massed artillery. There were periods in which rigid trench warfare broke down, such as during the Battle of the Somme, but the lines never moved very far. The war would be won by the side that was able to commit the last reserves to the Western Front.
John T. Thompson
World War I began in Europe in 1914, and Thompson was sympathetic to the Allied cause. Since the U.S. did not immediately enter the war, and because he recognized a significant need for small arms in Europe (as well as an opportunity to make a substantial profit), Thompson retired from the Army in November of that year and took a job as Chief Engineer of the Remington Arms Company. While with the company he supervised the construction of the Eddystone Plant in Chester, Pennsylvania, at that time the largest small arms plant in the world. It manufactured Pattern 1914 Enfield rifles for British forces, and Mosin–Nagant rifles for Russia.
The introduction of trench warfare in the First World War changed tactics substantially, and by 1916 Thompson was experimenting again with automatic small arms, this time attempting to design a weapon that troops could use to clear an enemy trench—what he called a "trench broom." Thompson studied several designs and was impressed with a delayed-blowback breech system designed by John Blish, a commander in the United States Navy. With Blish as a partner, Thompson obtained the necessary venture capital to form the Auto-Ordnance Company, and began working on the design of what eventually became the Thompson submachine gun.