Before the Khmer Rouge seized Cambodia in 1975 the US evacuated its embassy + offered to take high-ranking Cambodian officials out of the country. Most refused + were later executed, including Sirik Matak, whose letter to the US Ambassador could have been written by the Ukrainians today...
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Also, of interest: https://history.state.gov/historicaldocuments/frus1969-76v10/d234