It wasn't in WW2 that it was divided, but the Korean War that started in June of 1950 and it was divided to separate the fractions, but a peace treaty was never signed.
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USSR
Soviet Union
All areas outside of the Communist countries. South of the 38th Parallel in Korea. South of the 17th Parallel in Vietnam. West of the East/West German line, etc.
From 1954 to 1975, Vietnam was divided at the 17th parallel. In April, 1975, North Vietnam troops accepted the surrender of South Vietnam from its president, thereby unifying Vietnam into a single country.
The motivation was to: Scare the Japanese in to surrender quickly, such that there wouldn't be any need for the US military to invade the Japanese home islands (Something which would've been the most bloodiest campaign in the Second World war, that would've killed millions on both sides). Deter the Soviet Union. Show them who is boss. So that the USSR will not be motivated to take a piece of Japan or go south of the 38th parallel in the Korean peninsula or even decide to pick a fight with the US.