Kingdom of Cambodia (1975–76)

The Kingdom of Cambodia was an ephemeral state that existed between April 1975 and April 1976 and was ruled by Prince Norodom Sihanouk and the Khmer Rouge.

End of the civil war and fall of Phnom Pehn
Phnom Pehn fell on 17 April 1975 and a coalition of Sihanouk's Sangkum movement and the Khmer Rouges founded the Kingdom of Cambodia that replaced the Khmer Republic

Take over by Khmer Rouge and its aftermath
After Sihanouk resigned and went into exile on 2 April 1976, the Khmer Rouge remained in sole control and established the perhaps most totalitarian state in the history of mankind, Democratic Kampuchea.