This thesis provides a narrative of the complex political and military history of the civil war in Cambodia from 1979 to 1991, also known as the Third Indochina Conflict. The war started when communist Vietnam supported a Cambodian resistance army and defeated the Khmer Rouge communist government in 1979. The Vietnamese intervention in Cambodia drew ferocious opposition from regional powers and Cambodian resistance movements. As a result, China supported the communist resistance movement (Khmer Rouge) while the United States and the countries of the Association of South East Asian Nations supported two non-communist resistance movements. These three resistance movements joined forces to fight against the Vietnamese-installed government, the People's Republic of Kampuchea. The latter emerged victorious in 1991 due to three factors: an appealing political message, cohesive military organization, and the use of a hybrid army which was predominantly composed of territorial forces. Using the lessons from the civil war in Cambodia, this thesis provides insights into the complexity of civil war, the dynamics of hybrid warfare, and the challenges facing an insurgent movement which struggles to become an effective conventional army.