Answer: The United Nations established the 38th Parallel as the boundary between North and South Korea
Explanation:
The Korean War broke out in 1950 when the forces of Communist North Korea invaded South Korea. The United Nations condemned the invasion and sanctioned the U.S. to lead a force to retake South Korea.
They were able to do so but then China got involved and so both sides sued for peace. In the resulting agreement, the U.N established the 39th parallel as the border between the two nations and the area became a Demilitarized Zone.