To Assure and Conceal: Revisiting Secret Agreements (Mitsuyaku) in the U.S.-Japan Alliance

Informed by the concept of plausible deniability and newly-declassified U.S. and Japanese documents, this study explores the interconnectedness between public and private security assurances made during the 1957-1960 revision of the U.S.-Japan Security Treaty. The role of secret agreements is conceptualized as a form of covert operations in U.S.-Japan allied secret diplomacy. The revised security treaty and joint communiques announced by President Dwight D. Eisenhower and Prime Minister Nobusuke Kishi were reinforced by secret agreements. Dual confidential arrangements enabled the transit of nuclear-armed U.S. vessels and warplanes into Japanese territorial waters and airspace, along with the free-use of U.S. bases in Japan for Korean contingencies. The U.S. employed overt and covert mechanisms to preserve its extended deterrent capabilities in East Asia as well as to meet Congressional and military requirements to preserve U.S. base rights in Japan. Japanese officials utilized covert strategies, including concealing the existence of secret agreements, thereby denying alleged public deception and ensuring their political survival for decades. In essence, secret agreements lay at the heart of the U.S.-Japan asymmetric alliance.