Upon his discharge from the Marine Corps, Larsen enlisted in the United States Army. He went on to join the 75th Ranger Regiment, where he would remain for the next 14 years. Initially assigned to 1st Ranger Battalion at Hunter Army Airfield, Larsen parachuted into Panama with the Rangers during Operation Just Cause and was also involved in Ranger operations during the Gulf War. He began to involve himself more in combat sports and served as the president of the 1st Ranger Battalion's practical shooting club. Upon his reassignment to the 2nd Ranger Battalion, he started the Battalion's practical shooting club. He soon found himself as the Non-Commissioned Officer in Charge of combatives and Close Quarters Battle (CQB) training for the battalion. He utilized his martial arts training, having attained black belts in several disciplines including Brazilian Jiu-jitsu under Romero "Jacare" Cavalcanti and Russian Sambo, and merged them into a single, effective fighting style. As the program grew more elaborate, he became the NCOIC of combatives and CQB training for the entire 75th Ranger Regiment. During his service with the Rangers, he established himself as the Army's subject matter expert on combatives. When the opportunity to shape the Army's Combatives program came, he transferred to the Ranger Training Brigade, which was charged with the development of the combatives doctrine. During this time, he refined his training methods and started to compile a comprehensive training manual.