This paper explores the question of just war theory as it applies to the Korean War by relying on the work of Michael Walzer. In the first section, I discuss this issue with regards to the initiation of and intervention in the Korean War. North Korea violated the principles of just war by fabricating its invasion as a response to South Korean aggression. The U.S. intervention in the Korean War was a defensive war for the United States and the free world rather than a war defending South Korea alone. The U.S.S.R. violated legitimate procedures of just war, by covering up its involvement through deception. China’s intervention was based on presumptive assumptions that the war in North Korea was a threat to China’s state security and that the United States could attack China. In the second section, I examine the issue of justice in war conduct. Walzer emphasizes that the engaged states should seriously consider the means used to win a battle no less than the victory itself. This paper examines this issue by considering civilian casualties from U.S. air bombing and the execution of members of the National Guidance League (Gungmin Bodo Yeonmaeng). This paper concludes, based on the discussion of the above two issues, with a judgment on the responsibilities for the intervention in and the waging of the Korean War.