This is the third volume in an operational and chronological series covering the Marine Corps’ participation in the Vietnam War. This particular volume details the continued build-up in 1966 of the III Marine Amphibious Force in South Vietnam’s northernmost corps area, I Corps, and the accelerated tempo of fighting during the year—the result being an “expanding war.” Although written from the perspective of III MAF and the ground war in I Corps, the volume treats the activities of Marine advisors to the South Vietnamese Armed Forces, the Seventh Fleet Special Landing Force, and Marines on the staff of the U.S. Military Assistance Command, Vietnam, in Saigon. There are separate chapters on Marine air, artillery, and logistics. An attempt has been made to place the Marine role in relation to the overall effort.