Discussions about U.S. military posture in the Indo-Pacific often assume that the United States will have the ability to not only quickly access its military capabilities stationed in the region, but also to freely operate from bases in allied countries. The authors of this report explore this assumption, examining the opportunities and constraints that the U.S. military might face when operating from the territories of Japan, the Republic of Korea (ROK), and the Philippines. The authors examine the basing and access assumptions for the U.S. military should it wish to preposition supplies in, and operate from, these allies in peacetime and in a conflict over Taiwan when these allies themselves have not been attacked. For this research, the authors conducted a comprehensive literature review of historical and current studies on access; held an internal RAND workshop with military experts to determine the types of capabilities and access requests the United States might make of Japan, the ROK, and the Philippines in a Taiwan contingency; conducted extensive discussions and interviews in the fall of 2022 with officials and experts in Japan, the ROK, and the Philippines and with U.S. government personnel and experts in the United States who work on issues related to these three allies; and examined important agreements the United States has with each treaty ally that are relevant for U.S. military access and basing. The authors present their findings regarding access and basing for each ally and recommend ways to improve outcomes in both areas.