In the past decade, the U.S.-Japan alliance has weathered serious challenges and emerged stronger than ever. Its evolution since 2001 has surprised even hardened cynics as Japan embraced forward-leaning policies and more participation in regional and international security affairs. Despite these accomplishments, alliance managers cannot relax, however: new security challenges demand attention and responses and old alliance concerns – basic questions of deterrence, reassurance, and abandonment – continue to influence thinking in Washington and Tokyo.