Under President John F. Kennedy, US troop levels in Vietnam grew under the Military Assistance Advisory Group program from just under a thousand in 1959 to 16,000 in 1963. South Vietnamese President Ngo Dinh Diem's heavy-handed crackdown on Buddhist monks in 1963 led the US to endorse a deadly military coup against Diem. The war escalated further in 1964 following the controversial Gulf of Tonkin incident, in which a U.S. destroyer was alleged to have clashed with North Vietnamese fast attack craft. The Gulf of Tonkin Resolution gave President Lyndon B. Johnson broad authorization to increase U.S. military presence, deploying ground combat units for the first time and increasing troop levels to 184,000. Soviet leader Leonid Brezhnev responded by reversing Khrushchev's policy of disengagement and increasing aid to the North Vietnamese, hoping to entice the North from its pro-Chinese position. The USSR discouraged further escalation of the war, however, providing just enough military assistance to tie up American forces. From this point, the People's Army of Vietnam (PAVN), also known as the North Vietnamese Army (NVA) engaged in more conventional warfare with US and South Vietnamese forces.