Beginning in the 1990s, the United States and Japan began redefining their bilateral security alliance. This redefinition also prompted Japan to change its security policies. To what extent and in what ways have changes in the U.S.-Japan security alliance contributed to these changes? I argue that these changes are the result of negotiations of the Japan-U.S. Joint Declaration on Security Alliance for the 21st Century, signed on April 17, 1996, and the occurrence of an exogenous event. I also argue that the negotiated changes only progress to a certain point, at which time an exogenous event acts as a catalyst to continue change. I examine two cases of policy change associated with exogenous events (the U.S.-led invasion of Afghanistan and the 2003 North Korea Missile Crisis). Finally, I summarize my results that Japan was able to change its security policies through negotiations and through a series of systematic steps that follow a consistent trajectory of change; but exogenous events have continued the change that started as a result of the negotiations and the systematic steps. / Master of Arts