The Marine Corps served an important role in the Vietnam War taking part in such battles as Da Nang, Hue City, Con Thien and Khe Sanh. Individuals from the USMC operated in the Northern I Corps Regions of South Vietnam. While there, they were constantly engaged in a guerrilla war against the Viet Cong, along with an intermittent conventional war against the North Vietnamese Army (NVA). Portions of the Corps were responsible for the less-known Combined Action Program (CAP) that implemented unconventional techniques for counter-insurgency and worked as military advisers to the Republic of Vietnam Marine Corps. Marines were withdrawn in 1971, and returned briefly in 1975 to evacuate Saigon and attempt a rescue of the crew of the Mayagüez. Vietnam was the longest war for Marines; by its end, 13,091 had been killed in action, 51,392 had been wounded, and 57 Medals of Honor had been awarded. Due to policies concerning rotation, more Marines were deployed for service during Vietnam than World War II. While recovering from Vietnam, the Corps hit a detrimental low point in its service history caused by courts-martial and non-judicial punishments related partially to increased unauthorized absences and desertions during the war. Overhauling of the Corps began in the late 1970s, discharging the most delinquent, and once quality of new recruits improved, the Corps focused on reforming the NCO Corps, a vital functioning part of its forces.