When World War 2 ended, Korea was divided between the American-aligned Republic of Korea and the Soviet-aligned People's Democratic Republic of Korea; the rough terrain and strong guerilla presence did not allow Allied forces to push further into the Soviet-occupied zone. From 1948 and onwards, the ROK suffered from enormous pro-northern insurrections. By 1950, Tukhachevsky believed the time was right for Kim Il-Sung, Supreme Leader of the DPRK, to intervene; the Soviet nuclear program had matured, China has largely been reconstructed, and the US was pursuing a policy of demilitarisation, confident in the doctrine of mutually assured destruction.
On June 25th, 1950, the Korean People's Army began an all-out assault through the border, beginning the Korean War.