This paper traces the development of close air support (CAS) by the United States Marine Corps in World War II. The study examines how the Marines started developing their doctrine in the 1930s and adapted their (CAS) system based on the outcome of battles on Guadalcanal, Tarawa, Iwo Jima, and Okinawa, as well as during operations in support of the U.S. Army in the Philippines. Particular emphasis is placed on the development of Marine CAS doctrine, liaison organizational structures, aircraft, and air-to-ground weapons. This study is pertinent because it describes how the Marines developed a very effective weapon that greatly increased the potency of its amphibious operations. Additionally, this was initially accomplished during a period of very limited financial resources (before the start of World War II) and then limited time resources (during the war crisis). This study also shows how the Marines worked to support the forces on the ground with the best CAS system possible despite the opposition.