The Thompson submachine gun is a blowback-operated, selective-fire submachine gun invented and developed by Brigadier General John T. Thompson, a United States Army officer, in 1918. It was designed to break the stalemate of trench warfare of World War I, although early models did not arrive in time for actual combat. The Thompson saw early use by the United States Marine Corps during the Banana Wars, the United States Postal Inspection Service, the Irish Republican Army, and the Republic of China; the FBI, previously an unarmed agency, adopted their use following the 1933 Kansas City massacre.