 On 19 May 1966 the People's Army of Vietnam (PAVN) attacked the ARVN base at Gio Linh killing 43 and wounding 54.From 15–18 September 1966 the 1st Battalion, 26th Marines launched Operation Deckhouse IV and discovered that the PAVN had built a network of tunnels and bunkers in the Con Thien-Gio Linh area.Gio Linh was intended to form part of the McNamara Line and formed one corner of what became known as Leatherneck Square, with the other corners being Con Thien, Cam Lộ Combat Base and Đông Hà Combat Base.