The title of this work might suggest much ado about a very limited subject; it is not easy to cite offhand how the UN has been important in shaping Japanese security policies. Yet it turns out that a careful examination of Japanese debates about matters related to the UN reveals a great deal about the nuanced calculations of Japanese decision-makers. The study starts with Japanese bureaucrats exploring the possibility of using the newly established UN as a security institution for protecting a newly democratic Japan. The Korean War quickly left them disillusioned with the UN and forced them to accept the idea of a security treaty with the United States. But even in the negotiations for that treaty, the Japanese repeatedly proposed that the preamble should be the article of the UN Charter that gives legitimacy to individual and collective self-defense measures. In domestic Japanese politics, meanwhile, both the government and the opposition parties repeatedly treated the UN as a panacea. However, the Japanese have once again become disillusioned with the UN since their campaign to get a permanent seat on the Security Council failed to get adequate international support.