Korean war resulted from a lot of different things one being the influence by America in the South and the Soviet Union in the North. When they realised the soviets were taking Korea the American military quickly sent General Hodge and the 43rd infantry division to occupy the Southern half of Korea. Korea had been divided in two by two American officers without consulting the Soviets who actually were okay with the temporary division. Then began the influence and and the shaping of the two Koreas by the Super Powers. Americans had a more heavy hand in the shaping of the South Korean government by picking put who they wanted to work whilst in the North the Soviets were a little less intrusive but still guided Kim IL Sung to power because of his actions as a guerilla fighter during the war. Whilst the North purged Japanese collaborators the Americans chose to work with them. The problem was the American government had left all this work to an American general who had no idea about Korean culture, history, language or way of thinking so the poor guy was stumbling in the dark. By 1950 the South was plagued by political infighting and assassinations. Whilst Kim IL Sung had rapidly solidified his power through assassination and control over the various factions in North Korea. It wasn't the will of the Soviets to start the war in fact Kim IL Sung invaded without their full support.

Compared to the South the North was far more stable. The South barely had an army and the Americans prese present in Korea weren't properly equipped or reinforced. So when the North invaded on June 25th 1950 they were smashed back very quickly.

Source: A course in Modern Korean history Yongsei University Seoul.