During the appalling crimes of the Khmer Rouge the US remained vocally critical, but effectively stood back in as part of the push to open up China. The subject of human rights was never an issue of policy, not that it has ever been since. Washington postured over Pol Pot and the media moralised, there was a whitewash for who was funding Suharto's genocide in East Timor. It's impossible to understand these events without keeping in mind the American war on Vietnam. Originally the US waged war on South Vietnam after its proxy terror-state slaughtered 70,000 people and aroused an uncontainable resistance. The liberal administration in Washington arranged a "seat change" for President Diem, a man they had put in power, and just before JFK was assassinated Diem was promptly executed. The pretext for all of this was to contain China and the absurdity of this justification didn't become apparent until Nixon went to China years later. Nixon embraced Mao as US planes were dropping the equivalent of five Hiroshimas on Cambodia.
The US continued to block all attempts at peaceful settlement until 1975, by which time Cambodia and Vietnam had almost been bombed back into the stone age. Washington had failed to achieve its maximal objectives, a ravaged Indochina would live on, but its main objectives were actually fulfilled by 1965 - when the US installed Ferdinand Marcos in the Philippines and Suharto in Indonesia. By then Vietnam was devastated, estimates of the Vietnamese dead ranged from 3 to 4 million. The war had been continued for the sake of US prestige. The aim had been to kill the "virus" of independence in Vietnam, the fear was that the rot might spread to Indonesia and eventually Japan. That would mean the US would lose everything it had fought for in WW2. The common perception of American defeat in Vietnam is not unfounded. This reading lacks subtlety in terms of analysis and exhibits imperial self-pity in conclusion. In short, the US secured its regional objectives though Vietnam survived the onslaught as a outpost of Soviet power in the Far East.
The US establishment was left profoundly bitter and sick over Vietnam and remains so to this day. So when the Vietnamese invaded Cambodia the US led the chorus of condemnation, even though the Vietnamese drove out the Khmer Rouge. In fact, the US and its allies went as far as to support the Khmer Rouge as a government-in-exile of which Sihanouk became a representative. The Khmer Rouge were allowed to hold onto their seat in the UN as the legitimate representatives of Cambodia. And as a guerrilla force against the Vietnamese occupation the Khmer Rouge received training from the SAS, which the British lied about until 1991. Until then Western governments withheld a great deal of aid to starving Cambodia. The argument was that the Vietnamese would not allow food to reach everyone that it should, namely the Khmer Rouge - who were holding 45,000 people hostage at the Thai border with the protection of the UN. In spite of its battle to rebuild itself Vietnam sent over 25,000 tonnes of food aid across the border to feed civilians.
Upon the invasion Sihanouk was deployed to make a speech damning Vietnam's aggression at the UN, after which he sought refuge in China. He was later anointed as President of the coalition in waiting. As the respectable face of a government, which included many mass-murderers, Sihanouk thanked the US for passing on lists of Cambodian "traitors". In this grubby partnership Sihanouk was constantly aware that his life was in the hands of hardcore Khmer Rouge in his own entourage. In the meantime the Khmer Rouge decided to wage a guerrilla war and wait for the Vietnamese to be driven out of Cambodia as a force condemned as "outlaws" by the international community. The move would then be a forceful takeover of the country and a return to the barbarism of the 1970s. The Vietnamese occupation was not undeserving of criticism given its brutal repression of dissent and racist policies against minorities. In the end Vietnam left the country in the hands of Hun Sen, a former Khmer Rouge who had defected to Vietnam, who continues to dominate Cambodia.
With the end of the Vietnamese occupation the transition to a liberal democratic capitalist state began. Yet it was only a formal shift to a multiparty system of representation as Hun Sen remains in power to this day in a sort of dictatorship lite. The old command economy has been blown apart to make way for markets and the forces of capital. Sihanouk was reinstated as King while the Khmer Rouge stood at a far and tried to appear as a modern force by embracing market liberalism. The power-sharing arrangement was between Hun Sen and Prince Ranariddh and thankfully not Pol Pot. Once again the conservatives banded around the royal family began to collaborate with the Khmer Rouge. In response Hun Sen orchestrated a coup and purged around 40 royalists. By now King Sihanouk was fixing pardons to rehabilitate the old men who had led the Khmer Rouge in its heyday. It would be another decade until a handfull of these aging killers would even see an indictment.