<P> President Johnson had already appointed General William C. Westmoreland to succeed General Harkins as Commander of MACV in June 1964 . Under Westmoreland, the expansion of American troop strength in South Vietnam took place . American forces rose from 16,000 during 1964 to more than 553,000 by 1969 . With the U.S. decision to escalate its involvement, ANZUS Pact allies Australia and New Zealand agreed to contribute troops and matériel to the conflict . They were quickly joined by the Republic of Korea (second only to the Americans in troop strength), Thailand, and the Philippines . The U.S. paid for (through aid dollars) and logistically supplied all of the allied forces . </P> <P> Meanwhile, political affairs in Saigon were finally settling down--at least as far as the Americans were concerned . On February 14 the most recent military junta, the National Leadership Committee, installed Air Vice-Marshal Nguyễn Cao Kỳ as prime minister . In 1966, the junta selected General Nguyễn Văn Thiệu to run for president with Ky on the ballot as the vice-presidential candidate in the 1967 election . Thieu and Ky were elected and remained in office for the duration of the war . In the presidential election of 1971, Thieu ran for the presidency unopposed . With the installation of the Thieu and Ky government (the Second Republic), the U.S. had a pliable, stable, and semi-legitimate government in Saigon with which to deal . </P> <P> With the advent of Rolling Thunder, American airbases and facilities needed to be constructed and manned for the aerial effort . The defense of those bases would not be entrusted to the South Vietnamese . So, on March 8, 1965, 3,500 United States Marines came ashore at Da Nang as the first wave of U.S. combat troops into South Vietnam, adding to the 25,000 U.S. military advisers already in place . On May 5 the U.S. 173rd Airborne Brigade became the first U.S. Army ground unit committed to the conflict in South Vietnam . On August 18, Operation Starlite began as the first major U.S. ground operation, destroying an NLF stronghold in Quảng Ngãi Province . The NLF learned from their defeat and subsequently tried to avoid fighting an American - style ground war by reverting to small - unit guerrilla operations . </P> <P> The North Vietnamese had already sent units of their regular army into southern Vietnam beginning in late 1964 . Some officials in Hanoi had favored an immediate invasion of the South, and a plan was developed to use PAVN units to split southern Vietnam in half through the Central Highlands . The two imported adversaries first faced one another during Operation Silver Bayonet, better known as the Battle of the Ia Drang . During the savage fighting that took place, both sides learned important lessons . The North Vietnamese, who had taken horrendous casualties, began to adapt to the overwhelming American superiority in air mobility, supporting arms, and close air support by moving in as close as possible during confrontations, thereby negating the effects of the above . The Americans learned that PAVN (which was basically a light infantry force) was not a rag - tag band of guerrillas, but was instead a highly disciplined, proficient, and well motivated force . </P>

When did the us begin to fight in vietnam