After American troops withdrew, South Vietnam surrendered to North Vietnam.
The Vietnam war was due to Communism. The south Vietnam was against Communism, but the North of Vietnam were communists. They wanted Vietnam unified as a communist country.
America was helping the south fight, to prevent the communists from taking over. After the US left the war, the North and the South of Vietnam continued to fight till North Vietnam won the South.
The South surrendered and they were united as a communists state. They became the Socialist Republic of Vietnam.
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