By 1964, there were 23,000 U. S. troops in Vietnam, but this escalated further following the 1964 Gulf of Tonkin incident, in which a U. S. destroyer was alleged to have clashed with North Vietnamese fast attack craft. In response, 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.  Past 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. Every year onward there was significant build-up of US forces despite little progress, with Robert McNamara, one of the principal architects of the war, beginning to express doubts of victory by the end of 1966.  U. S. and South Vietnamese forces relied on air superiority and overwhelming firepower to conduct search and destroy operations, involving ground forces, artillery, and airstrikes. The U. S. conducted a large-scale strategic bombing campaign against North Vietnam. The Tet Offensive of 1968, proved to be the turning point of the war; despite years of American tutelage and aid the South Vietnamese forces were unable to withstand the communist offensive and the task fell to US forces instead. The Tet Offensive showed that the end of US involvement was not in sight, increasing domestic skepticism of the war. The unconventional and conventional capabilities of the Army of the Republic of Vietnam (ARVN) increased following a period of neglect and became modeled on heavy firepower-focused doctrines like US forces. Operations crossed international borders; bordering areas of Laos and Cambodia were used by North Vietnam as supply routes and were heavily bombed by U. S. forces.