Causes of the Korean War and the Influence of Joseph Stalin
USSR
CHINA
USA
The USSR was lead by Josef Stalin after WWII. Stalin had an interested in Korea since before WWII and wanted to try getting a foothold in it, but knew the threat of a war on two fronts, choosing to focus on Europe. Once the war ended, Stalin would again attempt a local present in Korea by supporting the North Korean military invasion of the South.
The People's Republic of China was in control by Mao Zedong after World War II and he was seeking an ally with the very powerful communist nation of the USSR as it would legitimize their "Continous Revolution".
The Truman administration was dedicated to spend less than previous ones, this was complicated as it also wanted to be involved in all world affairs after WWII. Korea became a matter of credibility for the United State's policy of world defense but evidence suggests that it's one they were not very interested in pursuing.
Korea
After their occupation by Japan, Korea's politics splintered into a modernized and liberalized version of Confucianism and Communism (both Maoism and Leninist). The nation was under the supervision of the United States and some Japanese were still left in governing positions. Korea's politics would futher faction into hundreds of small and ineffectual parties.