With the U.S.-China rivalry in Southeast Asia intensifying in recent years, the South China Sea has become a significant arena for strategic competition between the two great powers. As tensions over territorial disputes continue to destabilize the region, geopolitics has become more complex. The presence of the People’s Liberation Army (PLA) in disputed areas of the South China Sea has created operational challenges for the Association ofSoutheast Asian Nations (ASEAN), the United States, and U.S. allies and partners. Given that any instability in the region caused by Sino-U.S. rivalry will negatively affect ASEAN countries, defense diplomacy with both China and the United States is crucial for managing defense relations and cooperation.