On June 25, 1950, war broke out between North Korea and South Korea. This paper explores how and why the Western European and North American churches, their missionaries, and the Korean churches supported the South Korean government, the United States, and the United Nations during the Korean War. Christian churches in the Western nations considered it imperative to support the United Nations in its effort to maintain international order. Churches in Eastern Europe and China, however, considered the action of the United Nations as an attempt to oppose the liberation of Asian peoples. Due to the pro-American stance of the Christian churches the Communist regime of North Korea regarded Christianity as an anti-patriotic religion. Because of this belief, the North Korean regime intensified its anti-religious campaigns, and tried to wipe out Christians in the post-bellum period. Thus, the Korean War deepened the conflict between Marxist regime and Christian churches, and brought about an anti-religious campaign in North Korea and the transformation of North Korean churches into house churches.