In the inter-war years, United States Marine Corps Captain Evans F. Carlson went to China in 1937 and observed Communist 8th Route Army units of the National Revolutionary Army in action against the Imperial Japanese Army. Carlson and Merritt A. Edson are believed to have developed the fireteam concept during the United States occupation of Nicaragua (1912–1933). At that time the US Marine squad consisted of a Corporal and seven Marines all armed with a bolt-action M1903 Springfield rifle and an automatic rifleman armed with a Browning Automatic Rifle. The introduction of the Thompson submachine gun and Winchester Model 1912 shotgun was popular with the Marines as a point-defense weapon for countering ambush by Nicaraguan guerrillas within the thick vegetation that could provide cover for a quick overrun of a patrol. A team of four men armed with these weapons had proven more effective in terms of firepower and manoeuvrability than the standard nine-man rifle squad.