(CNN) -- At least 1.7 million people -- nearly a quarter of Cambodia's population -- were killed by execution, disease, starvation and overwork under the Khmer Rouge's brutal rule from 1975 to 1979.
Three of the ultra-Maoist regime's former leaders have been found guilty for crimes against community by a U.N.-backed tribunal.
On Thursday, guilty verdicts were handed down to Nuon Chea, otherwise known as Brother Number Two, and Khieu Samphan, the one-time President of Democratic Kampuchea, as Cambodia was known.
Kaing Guek Eav, alias Comrade Duch, the head of an infamous torture prison, was convicted of crimes against humanity in 2010. He was ultimately sentenced to life in prison in 2012, more than 30 years after the fall of the regime.
Here's a timeline of the Khmer Rouge regime and the long path to bring its leaders to justice:
1970: Marshal Lon Nol stages a coup, forcing Prince Norodom Sihanouk out of power. Sihanouk is aligned with the Khmer Rouge, an ultra-Maoist group building up a following in the countryside.
April 17, 1975: The Khmer Rouge seize Phnom Penh, completing their taking over of the country. The city's inhabitants are forced to leave the capital, heading out to rural areas. Some two million people are forced out of Cambodia's urban centers.
Late 1977: Fighting breaks out between Vietnam and Cambodia.
May 25, 1978: Khmer Rouge purges East Zone.
January 7, 1979: The Vietnamese take Phnom Penh, beginning 11 years of Vietnamese occupation. The Khmer Rouge move west. Some Cambodians celebrate January 7 as a liberation day from the Khmer Rouge, while others mark it as the start of Vietnamese occupation.
1979: A genocide tribunal in Phnom Penh finds Pol Pot and Ieng Sary guilty of genocide. Neither appeared in court or served any sentence
1982: Triparty coalition government forms, consisting of Prince Sihanouk, who was exile in China, the Khmer Rouge and non-communist leader Son Sann to create the Triparty Coalition Government. Vietnam helps establish a new government led by Heng Samrin.
1990: Vietnamese troops withdraw from Cambodia.
October 23, 1991: Paris peace talks. A peace accord among all Cambodian parties is signed. They approve holding a national election under the supervision of the United Nations Transitional Authority in Cambodia. The Khmer Rouge boycott the polls and won't demobilize their forces.
May 23-28, 1993: The U.N.-supervised elections are held. A royalist party lead by one of the king's sons, Ranariddh, wins a plurality and forms a coalition government with the Cambodian People's Party. Ranariddh and Hun Sen of the CPP are appointed co-prime minister. Khmer Rouge soldiers continue to wage guerrilla war.
1996: King Norodom Sihanouk pardons top cadre Ieng Sary's genocide sentence from the 1979 genocide tribunal.
1997: The Khmer Rouge tries Pol Pot for crimes allegedly committed within the regime after 1979. Cambodia asks the U.N. to help create a court to prosecute the surviving top leaders of the Khmer Rouge.
1998: Pol Pot dies. Civil war ends in Cambodia.
1999: Considered the end of the Khmer Rouge.
2001: Cambodia's National Assembly passes a law to create a court to try serious crimes committed during the Khmer Rouge regime.
2003: Agreement is reached with the U.N. on how the international community would help and participate in the Extraordinary Chambers in the Courts of Cambodia. The pact was amended in 2004.
June 2007: The court becomes fully operational.
November 20, 2007: The pre-trial chamber opens its first public hearing on an appeal by Duch, the Khmer Rouge head of the S-21 prison, where some 14,000 people died.
July 26, 2010: Duch is found guilty for war crimes, crimes against humanity, murder and torture, and sentenced to 35 years in prison.
February 3, 2012: Duch's sentence extended to life in prison.
August 7, 2014: Nuon Chea, otherwise known as Brother Number Two, and Khieu Samphan, the one-time President of Democratic Kampuchea, are found guilty and sentenced to life in prison.
Sources: Cambodia Tribunal Monitor (David Scheffer, Khamboly Dy), Extraordinary Chambers in the Courts of Cambodia, United Nations