Korean Reunification will transform overall military structure in Northeast Asia. Reduction of tension around the Korean Peninsula will impact on major countries’ military presence as well as their national security strategy. Especially, termination of nuclear danger by North Korea will reorganize nuclear policy of Six Party Talks countries which have challenged security environment in Northeast Asia. This book focuses on researching what is new military architecture in Northeast Asia after Korean Reunification and major countries’ security policy. Above all, Russia and China need to rebuild their national security strategy to cut down military expansion and Japan will finally repress its national ambition as a nuclear country. Then the Unified Korea will lead new security environment in Northeast Asia and U.S. military presence will be remained for regional security and peace. Therefore, this paper argues how Korean Reunification works out as the main key to reorganize military structure in Northeast Asia.