On February 19, 1965, some units of the Army of the Republic of Vietnam commanded by General Lm Vn Pht and Colonel Phm Ngc Tho launched a coup against General Nguyn Khnh, the head of South Vietnam's ruling military junta. Their aim was to install General Trn Thin Khim, a Khnh rival who had been sent to Washington D.C. as Ambassador to the United States to prevent him from seizing power. The attempted coup reached a stalemate, and although the trio did not take power, a group of officers led by General Nguyn Chnh Thi and Air Marshal Nguyn Cao K, and hostile to both the plot and to Khnh himself, were able to force a leadership change and take control themselves with the support of American officials, who had lost confidence in Khnh.

Although Khnh had seized power in January 1964 in alliance with Khim, the pair had soon fallen out over policy disputes along religious lines, and the Catholic Khim began to plot against Khnh. Khim was believed to have helped plan a failed coup in September 1964, and Khnh exiled him as a result. While in Washington, Khim continued to plot alongside his aide Tho, who was actually a communist agent bent on trying to foment infighting at every opportunity. Aware of Tho's plans, Khnh summoned him back to Vietnam in an apparent attempt to capture him, and Tho responded by going into hiding and preparing for his attack. In the meantime, Khnh's hold on power was slipping as his military support dwindled, and he became increasingly reliant on the support of civilian Buddhist activists who favored negotiations with the communists and opposed escalation of the Vietnam War. The Americansmost notably Ambassador Maxwell Taylorwere opposed to this and had been lobbying various senior Vietnamese officers such as K to overthrow Khnh, who knew that American-sponsored moves to depose him were afoot.

However, the Americans were not counting on Tho and his fellow Catholic Pht trying to seize power on an explicitly religious platform, claiming fidelity to slain former Catholic President Ng nh Dim and promising to recall Khim from the US to lead the new regime. This caused alarm among the Buddhist majority, who had campaigned heavily against Dim's discriminatory religious policies in the months leading up to his ouster in November 1963. Although they wanted Khnh gone, the Americans did not want Tho and Pht to succeed, so they sought out K and Thi in an attempt to have them defeat the original coup and then depose Khnh. During the initial attack, Tho and Pht tried to capture both Khnh and K, but both men escaped narrowly, although some of their colleagues in the Armed Forces Council were arrested. Although the rebels were able to take control of Tan Son Nhut Air Base, the largest in the country and the military headquarters of South Vietnam, K was able to regroup quickly and retain control of the nearby Bien Hoa Air Base, using it to mobilize air power and stop the rebel advance with threats of bombing. Late in the night, Tho and Pht met K in a meeting arranged by the Americans, where an agreement was reached for the coup to be ended in return for Khnh's ouster. By early next morning, the bloodless military action was over as Tho and Pht went into hiding, and the junta voted to sack their leader Khnh, who was absent on a military inspection tour, thinking that K and Thi were on his side.

When Khnh heard of his ouster, he declared it to be illegal. After defying his colleagues and travelling around the country for a day in a fruitless attempt to rally support for a comeback, Khnh went into exile after being named to fill the meaningless post of Ambassador-at-Large and allowed an elaborate ceremonial military send-off to save face. Pht and Tho were later sentenced to death in absentia. Tho was hunted down and killed in July 1965, while Pht remained on the run for several years before turning himself in and being pardoned.

The Army of the Republic of Vietnam (ARVN; Vietnamese: Lục quân Việt Nam Cộng hòa; French: Armée de la république du Viêt Nam) were the ground forces of the South Vietnamese military from its inception in 1955 to the Fall of Saigon in April 1975. It is estimated to have suffered 1,394,000 casualties (killed and wounded) during the Vietnam War.The ARVN began as a postcolonial army that was trained by and closely affiliated with the United States and had engaged in conflict since its inception. Several changes occurred throughout its lifetime, initially from a 'blocking-force' to a more modern conventional force using helicopter deployment in combat. During the American intervention, the ARVN was reduced to playing a defensive role with an incomplete modernisation, and transformed again following Vietnamization, it was upgeared, expanded, and reconstructed to fulfill the role of the departing American forces. By 1974, it had become much more effective with foremost counterinsurgency expert and Nixon adviser Robert Thompson noting that Regular Forces were very well-trained and second only to the American and Israeli forces in the Free World and with General Creighton Abrams remarking that 70% of units were on par with the US Army. However, the withdrawal of American forces by Vietnamization meant the armed forces could not effectively fulfill all of the aims of the program and had become completely dependent on U.S. equipment since it was meant to fulfill the departing role of the United States.At the ARVN's peak, an estimated 1 in 9 citizens of South Vietnam were enlisted, and it had become the fourth-largest army in the world composed of Regular Forces and the more voluntary Regional and Village-level militias.Unique in serving a dual military-civilian administrative purpose, in direct competition with the Viet Cong, the ARVN had also become a component of political power and suffered from continual issues of political loyalty appointments, corruption in leadership, factional infighting, and occasional open internal conflict.After the fall of Saigon to North Vietnam's People's Army of Vietnam (PAVN), the ARVN was dissolved. While some high-ranking officers had fled the country to the United States or elsewhere, thousands of former ARVN officers were sent to re-education camps by the communist government of the unified Socialist Republic of Vietnam. Five ARVN generals committed suicide to avoid capture by the PAVN/VC.