Japan's support would be the most important to the United States among its allies to defend Taiwan in the event of Chinese aggression, according to a U.S. think tank.

Given its geographic limitations, the U.S. would have to rely on support from allies in the Asia-Pacific region, and "above all Japan," the Council on Foreign Relations said in a recent report.

Japan hosts about 54,000 U.S. troops and the headquarters of some U.S. key units, such as the Navy's 7th Fleet and the 31st Marine Expeditionary Unit, the report noted.