> Japan has neither and it's dubious whether the United States would step in.

There is NO QUESTION the US would provide a full defense of Japan against any aggressive party.

The US has multiple military bases in Japan, with 35,000+ military personnel. Japan pays the US billions every year to support the US military presence there. Japan is also a too-big-to-fail economy (4th in the world) and US trading partner. And strategically, what do you think the US "pivot to Asia" means, if not defending close US allies in the Asia-Pacific from unprovoked aggression?

    For over 60 years the United States-Japan Alliance has served as the cornerstone of peace, stability, and freedom in the Indo-Pacific region.  The U.S. commitment to Japan’s defense under the U.S.-Japan Security Treaty of 1960 is unwavering. https://www.state.gov/u-s-security-cooperation-with-japan/