Now for the serious stuff, and another history lesson.
Just as with Vietnam and much of the surrounding area, Cambodia was colonised by the French in the mid-19th century before gaining independence in the 1950s. The period thereafter was one of peace and prosperity, and a time of creativity and optimism - Cambodia's golden years. Phnom Penh grew in size and stature, and the temples of Angkor (more on them in a few days) were the leading tourist destination in Southeast Asia. This, however, proved to be the calm before the storm.
The Khmer Rouge was an indigenous Cambodian revolutionary movement formed in the 1960s - a guerrilla group driven by communist ideals and led by a man known as Pol Pot. In March 1970 Cambodia's prime minister, Norodom Sihanouk, was deposed in a military coup and overthrown by General Lon Nol. Sihanouk took up residence in Beijing from where he set up a government-in-exile and formed an alliance with the Khmer Rouge for support. This gave their movement legitimacy, with Sihanouk becoming the nominal head of a Khmer Rouge. Sihanouk's popularity in rural Cambodia allowed the Khmer Rouge to extend its power and influence, and when Lon Nol's government collapsed the Khmer Rouge captured Phnom Penh on April 17th, 1975 and took over. That's when the shit hit the fan.
Upon taking power, Pol Pot and his Khmer Rouge immediately set about implementing one of the most radical and brutal restructurings of a society ever attempted. Its goal was to transform Cambodia into a giant peasant-dominated agrarian cooperative, untainted by anything that had come before. On that April 17th, the Khmer Rouge evacuated the entire population of Phnom Penh to camps in the countryside where they were forced to work as slaves for 12 to 15 hours a day (similar evacuations took place every time the Khmer Rouge took over a new city). Pol Pot envisioned a Cambodia absent of any social institutions like banks or religions or any modern technology. His means of implementation was to begin exterminating anyone who didn't fit this new ideal. Intellectuals, businessmen, Buddhists and foreigners were systematically wiped out, often by execution but sometimes simply by working people to death in the fields. This was pure genocide, akin to the Holocaust but at the hands of the Khmer Rouge and Pol Pot instead of Nazi Germany and Adolf Hitler. The advent of Khmer Rouge rule was even proclaimed "Year Zero".
The photos above and below are Tuol Sleng Genocide Museum. This is a former high school, taken over by Pol Pot's security forces who transformed the classrooms into torture chambers and renamed the facility Security Prison 21 (S-21).