Several hundred Cambodians and Buddhist monks gathered at the former execution ground to remember the day when 40 years ago the country fell to the Khmer Rouge.

The regime held power for four years during which an estimated 1.7 million Cambodians died from starvation, slave labor and wholesale murder and bloody purges.

After Khmer Rouge was overthrown in January 1979, the burial pits of the execution site were excavated and the skulls of the victims were put in a stupa as a reminder of what the country endured.

Forty years later and 6,000 miles away, John Gunther Dean, former US ambassador to Cambodia, recalls what he describes as one of the most tragic days of his life - April 12, 1975, the day the United States “abandoned Cambodia and handed it over to the butcher.”