After the outbreak of the Korean War on 25 June 1950, Kim became the commander of the I Corps. During the First Battle of Seoul, the I Corps held a position at Kumchon, North Hwanghae, to the north of Uijeongbu. Once he learned of the North Koreans' 28 June capture of Seoul, Kim had his forces retreat to the south bank of the Han River and set up a defensive line, which they were able to hold for six days until 3 July when North Korea had recovered sufficiently from the demolition of the Hangang Bridge to begin sending tanks across the river. Historian Jongsuk Chay credited the time earned by this operation with "saving the Republic of Korea from communism". Kim went on to fight in the Battle of Pusan Perimeter later that year. However, his conflicts with President Syngman Rhee and American military officers led to his discharge in March 1951.