Korean reunification selected the 38th parallel as a dividing line. Japanese troops to the North of this line were to surrender to the Soviet Union and troops to the South of this line would surrender to the United States. This was not originally intended to result in a long-lasting partition, but Cold War politics resulted in the establishment of two separate governments in the two zones in 1948 and rising tensions prevented cooperation. The desire of many Koreans for a peaceful unification was dashed when the Korean War broke out in 1950. In June 1950, troops from North Korea invaded South Korea.
Soviet Union in the Korean War both an unreliable and demanding ally and took greater steps to ensure autarky from the USSR in the years following the War. In 1960, China broke from the Soviet Union in an event known today as the Sino-Soviet Split, creating a rift amongst all communist powers which were expected to pick one side of the divide. The North Korean regime, which bordered both countries, successfully navigated the split by playing one power off against the other. It did not firmly align with either power until the Soviet reforms under Mikhail Gorbachev compelled the regime to align with China. This split,