By January 1944 the US Marine Corps had grown to a total of 405,169 personnel, comprising 28,193 officers, 10,723 officer candidates, and 366,353 enlisted men. The Fleet Marine Force now had two amphibious corps, four divisions, a separate infantry regiment, 19 defense battalions, and numerous support and service units. Following on from Battle Orders 1: USMC in the Pacific Theater of Operations 1941–43, this book examines the continuing development of the Corps's organization, its training, tactics, weaponry, and command structure, as well as the battles fought in the Southwest Pacific on New Britain, and in the Central Pacific on Tarawa, Roi-Namur, Eniwetok, Saipan, and Tinian. The organization of the 4th Marine Division (MarDiv) and III and V Amphibious Corps (IIIAC, VAC) is also discussed along with smaller, new units.