In 1970, the Cambodian Communists had few troops in the field and relied on the North Vietnamese to handle the brunt of the fighting. But as the war progressed, Khmer Rouge forces grew in number and battlefield prowess. They captured more and more territory on their own from Lon Nol’s army.
By the end of 1972, most North Vietnamese troops had left the country. Going forward, the Khmer Rouge relied mainly on China for weapons. The North Vietnamese, meanwhile, remained closely aligned with the Soviet Union. A growing rift between the world’s two Communist superpowers would soon be mirrored in divisions between the Cambodian and Vietnamese Communists.
As the Cambodian war continued, US involvement remained unpopular at home. By Congressional order, the aerial attacks ended in August 1973 after a final surge of bombing. But US weapons continued to flow to Lon Nol’s slowly retreating forces. Ultimately, civilian and military aid to his government totaled $1.6 billion.
In early 1975, as the Khmer Rouge conquered more territory and new waves of refugees swamped Phnom Penh, the White House lobbied Congress to authorize $220 million more in aid, in the hope that strengthened resistance would force the Khmer Rouge into a cease-fire and political settlement.
Congress refused. There was no settlement. On April 12, 1975, with Phnom Penh surrounded, US Marine helicopters evacuated American diplomats and a few Cambodians from the city.
Some historians contend that US military actions in Cambodia inadvertently strengthened the Khmer Rouge and facilitated their eventual victory. In this view, the bombing campaign drove Communist Vietnamese forces deeper into Cambodia and, by killing countless civilians, sowed widespread anger that helped the insurgents recruit supporters. Khmer Rouge fortunes were also helped by Vietnamese and Chinese aid, Sihanouk’s call to arms, and resentment of the widespread corruption that infected Lon Nol’s government and army.
US leaders at the time contended they were supporting a legitimate Cambodian government against aggression by Communist Vietnamese forces. Washington hoped that helping Lon Nol would ultimately serve the Cold War goal of stemming Communism’s spread in Southeast Asia.
In the end, a Communist movement that in 1970 had been small and largely powerless acquired strength and support sufficient to conquer the entire country.