The Khmer National Solidarity Party was formed by Khieu Samphan following his defection from the Khmer Rouge.
Following the fall of the Khmer Rouge and the Communist Party of Kampuchea, the new Vietnamese backed government reclaimed the history of the CPK and morphed into the Kampuchean Peoples Revolutionary Party.
The Khmer Rouge morphed into the Party of Democratic Kampuchea which officially at least renounced communism in favor democratic socialism. It was initially led by Pol Pot, but in 1985 Khieu Samphan became General-Secretary of the Party.
Considered a relative liberal (by the standards of the time) Khieu Samphan, had served in both the governments of King Sihanouk, as well as Democratic Kampuchea.
He led Party of Democratic Kampuchea as well as being part of the Coalition Government of Democratic Kampuchea .
In 1992 the Party of Democratic Kampuchea morphed into the Cambodian National Unity Party, with even vaguer goals and again with Khieu Samphan as their leader. The initial aim was to take part in the UN sponsored elections, which the Khmer Rouge refused to do, thus keeping the civil war going. It is estimated at the time that around 6% of Cambodian territory, including Pailin and Anlong Veng were still under KR control.
Following the defection of Ieng Sary in 1996 to form the Democratic National Union Party, Khieu stayed in charge for one more year, before also defecting.
Khieu formed the Khmer National Solidary Party, which initially planned to join the forces in opposition to the Cambodian Peoples Party, going so far as to state “If the elections do not go ahead under the iron rule of Vietnamese communists and their puppet, we should be very happy and want to take part because we are democratic, pluralistic and free government.”.
The CPP objected to this and for all intents the Khmer National Solidarity Party ceased to exist in any form of importance.
In 2018 Khieu Samphan was sentenced to life in prison and he is one of the most senior Khmer Rouge leaders still alive.