Japan ruled Korea between 1905-1945. In August 1945, during the closing days of WWII, the USSR invaded the peninsula and the Soviet 25th Army advanced as far south as Seoul, before pulling back to the 38th Parallel, which was the pre-agreed division point with the USA. On 8 September 1945 (six days after the surrender had been signed aboard USS Missouri), an advance guard of the US Army’s 40th Infantry Division landed in southern Korea and began to disarm the Japanese there. This split of the peninsula set the stage for the Korean War which lasted from June 1950 – July 1953.
The Japanese garrison in Korea had been relatively untouched by WWII until the final few weeks. As such, both the Soviets and the Americans captured vast quantities of equipment intact, which was then passed on to their respective North / South Korean client governments. The communists in the north were further aided by ex-Japanese weapons captured from the Japanese Kwangtung army and the puppet Manchukuo army during the short Soviet operation at the close of the war. Unwanted by the USSR, these weapons were transferred to either the North Koreans or Mao’s communist Chinese forces. The Chinese in turn re-transferred some Japanese equipment to the North Koreans, besides using it themselves.
(Abandoned Ki-51 “Sonia” dive bombers at the former Imperial Japanese Army’s Pyeongtayk airbase in the autumn of 1945. This was one of the first Japanese army airfields in Korea, opening in 1919. During the Korean War, this airbase was refurbished and was codenamed K-6. After the war it was renamed Camp Humphreys and today it is home of the 6th Air Cavalry Brigade. As for the “Sonia”, this was an obsolete design and none of the planes above were ever restored.)
(A Tachikawa Ki-9 “Spruce” of the South Korean air force at airbase K-1 near Busan, South Korea, in 1951. This obsolete biplane had been an intermediate trainer of the Imperial Japanese Army Air Force during WWII. A large number were recovered intact by both the North and South Koreans and both air forces used the “Spruce” during the Korean War.)