Many people were surprised by this answer; see for example Nick's blog.
The division of Korea was one of the conditions of the Japanese surrender at the end of World War II (approved on 17 August 1945, and effective from 2 September). Korea had been under Japanese rule since 1910. Following the surrender, Japanese forces to the north of the 38th Parallel were ordered to surrender to the Commander in Chief of Soviet Forces in the Far East, and those to the south were directed to surrender to the Commander in Chief of US Army Forces in the Pacific.
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