The correct answer to this open question is the following.
The two governments of Vietnam reacted following the free elections in that North Vietnam launched an invasion of South Vietnam.
The Geneva accords of July 1945 had established the division between North and South Vietnam. The dividing line was Parallel 17th. The agreement established that free elections should be held in two years to unify the country, but it never happened. Ngo Ding Diem leads South Vietnam and Chi Minh rules in North Vietnam. Diem is supported by the United States that want to stop the spread of Communism and North Vietnam is backed by the Soviet Union that wants to spread Communism in the region.