The Que Son Valley is located along the border of Quang Nam and Quang Tin provinces. During the Vietnam War it lay in the southern part of South Vietnam's I Corps. Populous and rice-rich, the valley was viewed as one of the keys to controlling South Vietnam's five northern provinces by the North Vietnamese and by early 1967 at least two regiments of the PAVN 2nd Division had been infiltrated into the area. The Quế Sơn Valley was also recognized as strategically important by the U.S. Military Assistance Command, Vietnam (MACV). The 1st Battalion, 5th Marines and 3rd Battalion, 5th Marines, experienced units that had fought in Vietnam since their arrival in the summer of 1966, were assigned to the valley in 1967 to support the outnumbered the Army of the Republic of Vietnam (ARVN) forces in the area.
During Operation Union from 21 April to 16 May, the 3rd Battalion, 1st Marines had fought the PAVN 21st Regiment near the Marine outpost on
Loc Son Mountain for control of the southern part of the Quế Sơn Valley. Operation Union II was launched on May 26 to destroy the withdrawing remnants of the PAVN with a helicopter assault by the 5th Marine Regiment, commanded by Colonel Kenneth Houghton. The assault was coordinated with ground attacks by the ARVN 6th Regiment, 2nd Division and the 1st Ranger Group.