To support South Vietnam's struggle against the communist insurgency, the United States began increasing its contribution of military advisers, using the controversial 1964 Tonkin Gulf incident as a pretext for such intervention. US forces became embroiled in ground combat operations in 1965, and at their peak they numbered more than 500,000. Communist forces attacked major targets in South Vietnam en masse during the 1968 Tet Offensive, and although their campaign failed militarily, it shocked the American establishment, and turned US public opinion against the war. Communist forces supplying the Vietcong carried supplies along the Ho Chi Minh trail, which passed through Laos and Cambodia. US presidents Lyndon Johnson and Richard Nixon authorized Operation Commando Hunt and Operation Menu, SAC bombing campaigns in Laos and Cambodia, about which only high-ranking Congressional officials were informed.