He was a consistent critic of American foreign policy, a campaigner for Asian communist and socialist movements and a supporter of the Khmer Rouge.

In 1978 Caldwell was one of the Labour Party candidates in St Mary's ward in the local elections for the Bexley London Borough Council.

He frequently attempted to downplay reports of mass executions by the Khmer Rouge in Cambodia, and was widely criticised by numerous authorities for doing so.

[5] In December 1978, Caldwell was a member, along with Elizabeth Becker and Richard Dudman, of one of the few groups of Western journalists and writers invited to visit Cambodia since the Khmer Rouge had taken power in April 1975.

"[9] The Khmer Rouge conducted an internal inquiry, which found that one of the guards present "had been having an unhappy love affair",[9] and had gone on a shooting spree before killing himself.

Pol's former aide and "Head of Security at the Foreign Ministry",[9] Phi Phuon, who witnessed the aftermath of the incident, thought that the purported killer "had been murdered and someone had tried to mask his death as a suicide.

"[10] However, journalist Andrew Anthony wrote that this "seems unlikely",[10] citing historian David P. Chandler, who met the translator present at Pol and Caldwell's meeting, "who remembered a very pleasant exchange conducted in a spirit of enthusiastic agreement.

"[10] In his biography of Pol Pot, author Philip Short claimed that the "likeliest explanation, which, perversely, the regime refused to credit because of its obsession with traitors, was that the attack was the work of a Vietnamese commando unit.

"[9] Three days after Caldwell was killed, the Vietnamese invaded Cambodia and soon put an end to the Khmer Rouge government.