A trove of more than 500,000 US diplomatic cables from 1978 released by WikiLeaks on Wednesday includes hundreds that paint a vivid picture of a US administration torn between revulsion at the brutality of Pol Pot’s government and fear of Vietnamese influence should it collapse.

“We believe a national Cambodia must exist even though we believe the Pol Pot regime is the world’s worst violator of human rights,” reads a cable sent by the State Department to six US embassies in Asia on October 11, 1978. “We cannot support [the] Pol Pot government, but an independent Kampuchea must exist.”

They are the second batch of cables to be released by the whistle-blowing website from Jimmy Carter’s presidency, which was marked by a vocal emphasis on human rights. That focus shines through in much of the correspondence, even to the point of wishing success on the Khmer Rouge in repelling Vietnamese incursions during the ongoing war between the two countries, in the hope it would, paradoxically, prevent more of the worst excesses of the government in Phnom Penh.

“While the Pol Pot government has few, if any, redeeming features, the cause of human rights is not likely to be served by the continuation of fighting between the Vietnamese and the government,” reads a cable sent by the US Embassy in Thailand to the State Department on October 17. “A negotiated settlement of [Vietnamese-Cambodian] differences might reduce the purges.”

In relation to that war, the cables give insight into some of the tactics employed by the Vietnamese to topple the Khmer Rouge, including allegedly training up Cambodians to operate as subversives in their homeland. They also reported the Khmer Rouge assertion that the murder of British academic and Khmer Rouge sympathiser Malcolm Caldwell during a trip to Cambodia was perpetrated by infiltrating enemy agents. Elsewhere in the cable, they relayed reports that the killers had been dressed differently from most cadres, and noted that, at the very least, the killing would be great propaganda for the Vietnamese.

“That such an incident took place in their presumably heavily guarded compound in Phnom Penh will be a propaganda coup for the Vietnamese, who have been harping for months on how insecure the Pol Pot government is,” reads a cable from the China US Liason Office to the State Department on December 23, the day of Caldwell’s death.

The cables also present a picture of aggressive denials from Chinese officials in the face of US accusations of atrocities carried out by the Khmer Rogue, including a fiery meeting between a Chinese delegation and US senators in August, in which Chinese Ambassador to the United States Chai Tse-Min told Senator John Sparkman “reports of mass killings in Cambodia were untrue”.

They also highlight how Chinese support for Cambodia was driven by fear of Vietnamese expansionism in the region.

“[Chinese foreign minister] Huang Hua noted that having defeated the US and acquired large quantities of US arms, the Vietnamese had ‘swollen heads’, and that they have long harbored plans for an Indochinese Federation,” reads a cable from the US Embassy in Malaysia to the State Department on April 27, sent after a meeting with a member of a Swedish government delegation that had just visited China.

Yet while the US government was aware of the horrific actions of the Pol Pot regime, with a July 21 cable from the US Embassy in Laos estimating 2 million people had died at its hands, it refused overtures from the country’s previous leadership to challenge the Pol Pot government’s right to represent Cambodia at the United Nations.

“The US is most sympathetic to the human rights concerns that have moved the Lol Non delegation to issue a credentials challenge to the [Government of Democratic Kampuchea] delegation,” reads a July 20 cable from the State Department to various embassies. “However [the US’s UN delegation] should not support a credentials challenge.”

While that refusal is based on the fact the US considered the “participating government” to be “the government which is in de facto control of the country, unless the government was imposed from outside”, other cables point to the administration’s deep fear of instability should the Khmer Rouge fall.

“If the Pol Pot regime was toppled, this could result in indefinite guerrilla warfare in Cambodia,” reads a cable from the State Department to the UN on December 16.

Nine days later, the Vietnamese launched a full-scale invasion of the country, forcing the Khmer Rouge to retreat to ever-more isolated redoubts before their final surrender in the late 1990s.

ADDITIONAL REPORTING BY SEAN TEEHAN