Fighting on Allies' Territory Overseas: Challenges and Opportunities for the United States in the Indo-Pacific Region

Key Chapter Title List

1. Introduction
2. Japan
3. Republic of Korea
4. Philippines
5. Conclusions and Recommendations
6. Setting the Operational Environment
7. List of Abbreviations
8. References

Document Introduction

Over the past two decades, China's significant rise in power and military capabilities, along with its coercive behavior towards other countries regarding Taiwan and in the Indo-Pacific region, has underscored the importance of the U.S. military maintaining appropriate peacetime posture and the ability to respond rapidly in a conflict. A core assumption implicit in many U.S. military and policy discussions about regional posture is that the United States can not only rapidly employ its forces already in the region but also freely use bases on allied territory for operations. This report examines the validity of this assumption by focusing on three U.S. treaty allies—Japan, the Republic of Korea (ROK), and the Philippines—analyzing the opportunities and constraints the United States might face when operating from their territories.

The report's core research questions include: whether existing security agreements provide the access the United States needs from each ally; whether the U.S. and its allies interpret these agreements consistently to minimize implementation friction; the process for introducing new U.S. capabilities; the impact of non-legal factors such as political considerations and public opinion on access; the treaty invocation process and the effectiveness of access guarantees; and differences in access challenges during peacetime versus contingencies.

To address these questions, the research employed a three-phase methodology: First, a comprehensive literature review of historical and existing relevant studies, war games, and simulation summaries. Second, internal expert workshops to identify the specific capabilities and access the United States might request from the three countries in a Taiwan contingency, covering categories such as logistics and supply, munitions, non-kinetic forces, kinetic ground equipment, combat aircraft, and naval vessels. Finally, based on the findings from the first two phases, extensive discussions with officials and experts from the three countries, as well as relevant U.S. government personnel and experts, along with in-depth analysis of the core security agreements between the United States and each ally.

The study found that, while the bilateral agreements with the three allies differ, none clearly define the rules for U.S. military access in a conflict where the ally itself is not under attack; domestic political considerations are the most critical factor determining access in both peace and conflict; and differing interpretations of core agreements can lead to divergent expectations. In a Taiwan conflict scenario, Japan is the most likely to permit a wide range of U.S. operations, the ROK is highly unlikely to grant access due to its focus on the North Korean threat and desire to avoid entanglement in a U.S.-China conflict, and the Philippines' decision would heavily depend on whether its territory or interests are directly threatened. The report concludes with targeted recommendations for the U.S. military and government, including planning for uncertainty, enhancing bilateral communication, optimizing capability deployment, and clarifying agreement provisions, to inform U.S. military access strategy in the Indo-Pacific region.