Were America to weaken significantly and to decrease its strong military commitment to Japan, Tokyo would likely increase defense spending significantly and shelve Article 9 of its 1946 constitution, which states that “land, sea and air forces, as well as any other war potential, will never be maintained.”
At present, Washington does want Japan to do more to balance China, but US withdrawal from Asia could trigger a regional arms race, leaving all countries less secure and possibly hurting regional and global economic stability.