The US-Japan and US-Republic of Korea (ROK) alliances have long been the pillars of regional stability in the Asia-Pacific, enabling a peaceful environment for East Asia’s remarkable economic development and integration. These alliances represent not only the full commitment of the US to the defense of Japan and the ROK, but also the willingness of Japan and the ROK to play an active role in regional and global security. In particular, these alliances are necessary to deter aggression from the Democratic People’s Republic of Korea (DPRK) and elicit China’s cooperation in contributing to regional stability and prosperity. However, bifurcated security arrangements alone are no longer enough to meet today’s geopolitical security challenges. A strong trilateral relationship is required that needs to be based on not only strengthened bilateral ties between the US and its allies but also between Japan and ROK. This volume assesses challenges and opportunities within the three bilateral relationships by focusing on areas of mistrust and providing concrete solutions to strengthen cooperation.