The correct answer is "Cambodia invaded South Vietnam."
The Vietnam War was a war fought in the countries of Vietnam, Laos, and Cambodia from November 1st, 1955 to April 30th, 1975. The territory of Vietnam was divided into 2 administrations. The northern part was ruled by a communist regime supported by China, whereas the southern part was governed by a regime which was friendly towards western influences. After numerous years in combat, the US Army ultimately decided to retreat from the territory. This meant the subsequent invasion of Saigon by communist-led forces and the conversion of the countries of Cambodia, Laos, and Vietnam towards Communism.