This is the second volume in a series of chronological histories prepared by the Marine Corps History and Museums Division to cover the entire span of Marine Corps involvement in the Vietnam War. This volume details the Marine activities during 1965, the year the war escalated and major American combat units were committed to the conflict. The narrative traces the landing of the nearly 5,000-man 9th Marine Expeditionary Brigade and its transformation into the ΙII Marine Amphibious Force, which by the end of the year contained over 38,000 Marines. During this period, the Marines established three enclaves in South Vietnam’s northernmost corps area, I Corps, and their mission expanded from defense of the Da Nang Airbase to a balanced strategy involving base defense, offensive operations, and pacification. This volume continues to treat the activities of Marine advisors to the South Vietnamese armed forces but in less detail than its predecessor volume, U.S. Marines in Vietnam, 1954-1964; The Advisory and Combat Assistance Era.