1965 battle of the Vietnam War
Source (Wikipedia): Battle of Ia Drang

The Battle of Ia Drang was the first major battle between the United States Army and the People’s Army of Vietnam (PAVN), as part of the Pleiku campaign conducted early in the Vietnam War, at the eastern foot of the Chu Pong Massif in the central highlands of Vietnam, in 1965. It is notable for being the first large scale helicopter air assault and also the first use of Boeing B-52 Stratofortress strategic bombers in a tactical support role. Ia Drang set the blueprint for the Vietnam War with the Americans relying on air mobility, artillery fire and close air support, while the PAVN neutralized that firepower by quickly engaging American forces at very close range.