Complex sentence: The 5th Division was reactivated in 1966 during the Vietnam War where its 13th, 26th, and 27th regiments under the command of the 3rd Marine Division (Reinforced) and 1st Marine Division (Reinforced) saw action in Vietnam until 19 March 1970.
Split into simpler sentences: The 5th Division was reactivated in 1966 during the Vietnam War. In Vietnam, its 13th, 26th, and 27th regiments were under the command of the 3rd Marine Division (Reinforced) and 1st Marine Division (Reinforced) until 19 March 1970.