Japan is the third-largest economy in the world and a key ally of the United States. Yet the determinants of Japanese security policy are not well understood. The question of why Japan never sought the independent military capabilities that would be commensurate with its economic power has puzzled scholars of international relations for decades. Applying new tools for the quantitative analysis of text to a new collection of 7,497 Japanese-language election manifestos used in elections between 1986 and 2009, this book argues that the electoral strategies politicians in the ruling party were forced to adopt under Japan's old electoral system made it extraordinarily difficult for them to focus on security issues and to change security policy. It was only when their electoral strategies shifted after electoral reform in 1994 that these same politicians became able to pay attention and change security policy.