The Khmer Rouge were extremist communists who came to power in 1975. They began a mass social engineering project forcing millions of people into brutal labour camps and executed professionals and intellectuals. By the time they were defeated in 1979, around two million people had been killed, about a quarter of the population. Sokphal Din was a teenager who survived ‘the killing fields’. He told Witness History about that terrifying ordeal, and how it could happen again if the lessons from Cambodia aren’t learnt.