Green = South Korea
Red = North Korea, China, and USSR.
The Korean War was a war between the Republic of Korea (supported primarily by the United States of America and the United Nations) and the Democratic People's Republic of Korea - supported by the People's Republic of China, with military and material aid from the Soviet Union.
The United States, worried about whether Japan would surrender through the threat of more atomic bombs being dropped, argued for a Soviet invasion of Japan and its colonial possessions. The Soviet Union easily agreed, smashing the remnant Kwantung Army that controlled most of the Japanese mainland.
Soon, the Manchuria (northeastern Chinese provinces like northern Inner Mongolia, Heilongjiang, Jilin, and Liaoning) was under Soviet control. The Soviets agreed to stop their advance at the 38th Parallel. Surprisingly, they actually did stop at the 38th Parallel, though many people agree that it was because they believed they could gain more concessions in Europe if they did so - and they cared more about Europe than a small peninsula in the middle of East Asia.
American forces secured South Korea, including several places that were not connected to the South Korean mainland. The People's Republic of Korea had been established with the approval of former Imperial Japanese administrators on September 6, 1945. Since the new nation was strongly Socialist, planning to nationalize the mining, transportation, banking, and communication industries already eight days after its creation, the United States Army Military Government in Korea was formed by arriving American forces.
In the Moscow Conference in 1945, Korea was to be controlled by a U.S.-Soviet Joint Commission. After five years, Korea would be free. However, most Koreans disliked the idea, including Syngman Rhee. Strikes and civil disobedience spread rapidly around Korea. The Military Government declared martial rule and held an election under UN auspices for independence. The Soviets and Korean Communists disagreed. The Republic of Korea was established on August 15, 1948, with Syngman Rhee becoming President. The Soviets established the Democratic People's Republic of Korea, controlled by Kim Il-Sung.
After the Chinese Civil War, where tens of thousands of North Koreans served with the Communists, Mao pledged aid to the North Koreans if war broke out against the south. Kim Il-Sung asked for Soviet support for an invasion of South Korea as early as March 1949. For the next two years, Stalin provided weapons to North Korea. Finally, in April 1950, Kim received permission to invade the south. The war began on June 25, 1950.
On June 25, 1950, the Korean People's Army (KPA) crossed the 38th Parallel. The South Koreans had no tanks, anti-tank weapons, or heavy artillery to counter Northern tanks and heavy artillery. Most of their soldiers were lost in the first few days. Just two days later, large parts of the South Korean government evacuated Seoul, while the bridge across the Han River was blown up on the next day, June 28. Still, the North Korean army captured Seoul on that same day.
The Soviets, a permanent member of the UN Security Council (UNSC), had boycotted the meetings ever since January 1950 because Taiwan held China's permanent seat on the UNSC, allowing the UNSC to unanimously condemn the North Korean invasion. Resolution 83 was passed encouraging UN member states to send military aid to Korea. The North Koreans were able to push back American-Korean forces to the areas surrounding Pusan, massacring civil servants and intellectuals along the way. General Macarthur commanded an amphibious landing at Incheon on September 15. Seoul was taken quickly on September 25. North Korean troops began dissolving.
Chinese troops secretly crossed the Yalu River (border between China and Korea), UN forces retreated to the Suwon-Wonju-Samcheok line. In 1952, Dwight D. Eisenhower was elected president. An Armistice was proposed by India, and a ceasefire was established, along with the Korean Demilitarized Zone (DMZ), patrolled by most of the belligerents. There was no peace treaty, but functionally, the Korean War is over.