Vietnam ground combat operations in 1965, and at their peak they numbered more than 500,000. The US also engaged in a sustained aerial bombing campaign. Meanwhile, China and the Soviet Union provided North Vietnam with significant material aid and 15,000 combat advisers. Communist forces supplying the Việt Cộng carried supplies along the Hồ Chí Minh trail, which passed through the Kingdom of Laos. The communists attacked South Vietnamese targets during the 1968 Tết Offensive. Although the campaign failed militarily, it shocked the American establishment, and turned US public opinion against the war. During the offensive, communist troops massacred over 3,000 civilians
South Vietnam in 1971. In accordance with the Paris Peace Accords signed with North Vietnam on 27 January 1973, US military forces withdrew from South Vietnam. North Vietnam was allowed to continue supplying communist troops in the South, but only to the extent of replacing materials that were consumed. The communist leaders had expected that the ceasefire terms would favour their side. But as Saigon began to roll back the Viet Cong, they found it necessary to adopt a new strategy, hammered out at a series of meetings in Hanoi in March 1973, according to the memoirs of Trần Văn Trà. As