Trench warfare
Trench warfare occurred when a revolution in firepower was not matched by similar advances in mobility, resulting in a grueling form of warfare in which the defender held the advantage. On the Western Front in 1914–1918, both sides constructed elaborate trench, underground, and dugout systems opposing each other along a front, protected from assault by barbed wire, mines, camouflaged trapping pits, and other obstacles. The area between opposing trench lines (known as "no man's land") was fully exposed to artillery fire from both sides. Attacks, even if successful, often sustained severe casualties.
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.