The Khmer Rouge was the ruling party in Cambodia from 1975 to 1979, but during this short time it was responsible for one of the worst mass killings of the 20th Century.
More than a million people died under the four-year regime
The brutal regime claimed the lives of more than a million people - and some estimates say up to 2.5 million perished.
Under the Marxist leader Pol Pot, the Khmer Rouge tried to take Cambodia back to the Middle Ages, forcing millions of people from the cities to work on communal farms in the countryside.
But this dramatic attempt at social engineering had a terrible cost, and whole families died from execution, starvation, disease and overwork.
The Khmer Rouge had its origins in the 1960s, as the armed wing of the Communist Party of Kampuchea - the name the Communists used for Cambodia.
Based in remote jungle and mountain areas in the north-east of the country, the group initially made little headway.
But after a right-wing military coup toppled head of state Prince Norodom Sihanouk in 1970, the Khmer Rouge entered into a political coalition with him and began to attract increasing support.
The Khmer Rouge expanded their reach from the remote north-east
In a civil war that continued for nearly five years, it gradually increased its control in the countryside.
Khmer Rouge forces finally took over the capital, Phnom Penh, and therefore the nation as a whole in 1975.
During his time in the remote north-east, Pol Pot had been influenced by the surrounding hill tribes, who were self-sufficient in their communal living, had no use for money and were "untainted" by Buddhism.
When he came to power, he and his henchmen quickly set about transforming Cambodia - now re-named Kampuchea - into what they hoped would be an agrarian utopia.
Declaring that the nation would start again at "Year Zero", Pol Pot isolated his people from the rest of the world and set about emptying the cities, abolishing money, private property and religion, and setting up rural collectives.
Anyone thought to be an intellectual of any sort was killed. Often people were condemned for wearing glasses or knowing a foreign language.
Hundreds of thousands of the educated middle-classes were tortured and executed in special centres.
Pol Pot led the brutal regime, but died without being brought to justice
The most notorious of these centres was the S21 jail in Phnom Penh, where more than 17,000 men, women and children were imprisoned during the regime's four years in power.
Hundreds of thousands of others died from disease, starvation or exhaustion as members of the Khmer Rouge - often just teenagers themselves - forced people to do back-breaking work.
The Khmer Rouge government was finally overthrown in 1979 by invading Vietnamese troops, after a series of violent border confrontations.
The higher echelons of the party retreated to remote areas of the country, where they remained active for a while but gradually became less and less powerful.
In the years that followed, as Cambodia began the process of reopening to the international community, the full horrors of the regime became apparent.
Survivors told their stories to shocked audiences, and in the 1980s the Hollywood movie The Killing Fields brought the plight of the Khmer Rouge victims to worldwide attention.
Pol Pot was denounced by his former comrades in a show trial in July 1997, and sentenced to house arrest in his jungle home.
But less than a year later he was dead - denying the millions of people who were affected by this brutal regime the chance to bring him to justice.