On April 17,1975, a hardline Communist command led by Pol Pot swept into Phnom Penh and overthrew the US-backed military government of General Lon Nol. The Khmer Rouge went on to write the darkest chapter in Cambodia's history, committing one of the worst genocides of the 20th century. More than two million people were killed during their four-year rule, as they carried out mass killings and forced labour in their pursuit of radical ideologies.