Due to the geographical proximity and history, the Korean peninsula has traditionally been and continues to be one of the primary security interests for Japan. The Korean peninsula has been the strategic pivot in Northeast Asia and would directly affect the course of Japan’s own security. The memoirs of Munemitsu Mutsu (late ninteenth-century Meiji-era diplomat) Kenkenroku, a realist classic in understanding Japanese diplomacy toward Korea, depicted how Japan struggled in dealing with Korea during the Sino-Japanese War over the Korean peninsula in the transition to the modern era.1 As one Korea security expert and former military intelligence officer noted, the relationship between Japan and the Korean peninsula has traditionally been described as “shinshi-hosha” in Japanese, “lips and teeth, cheekbone and jawbone” in English, taken from an ancient Chinese phrase—which meant a relationship so mutually dependent that if one falls the other falls with it.2 Since the defeat of Japan in the Second World War and the end of the colonial period, Japan has allied with the United States to deal with security on the Korean peninsula and Northeast Asia.