In 1963, Buddhist discontent with Diệm's Catholic regime erupted into mass demonstrations, leading to a violent government crackdown. This led to the collapse of Diệm's relationship with the United States, and ultimately to a 1963 coup in which he and Nhu were assassinated. The Diệm era was followed by more than a dozen successive military governments, before the pairing of Air Marshal Nguyễn Cao Kỳ and General Nguyễn Văn Thiệu took control in mid-1965. Thiệu gradually outmaneuvered Kỳ and cemented his grip on power in fraudulent elections in 1967 and 1971. During this political instability, the communists began to gain ground. To support South Vietnam's struggle against the communist insurgency, the United States began increasing its contribution of military advisers, using the 1964 Gulf of Tonkin incident as a pretext for such intervention. US forces became involved in ground combat operations by 1965, and at their peak several years later, 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 Laos.