Nipponica

While Japan can never become an official member of the regional alliance, its participation in the organization's activities and cooperation with its member states is increasingly important as it develops its defense capacity.

There are limits to Japan's partnership with NATO. The organization cannot come to the defense of Japan, even if it were to enter conflict with Russia, China, or North Korea. But NATO's member states, especially the U.S., could certainly support Japan with military and nonmilitary support, if necessary. Beyond the mutual defense agreement of the NATO treaty (Article 5), there are other aspects of the treaty that allow for institution building and capacity training between NATO, its members and Japan.

This is a good explainer article about how Japan fits into the western security umbrella and how it could come to the nation’s aid in the event of conflict.