Some of the most enduring disagreements in the alliance between the United States and the Republic of Korea (ROK) concern ballistic missile defenses (BMD). At the same time that South Korea has expanded its conventional offensive missile program, it has declined American proposals for a regionally integrated BMD architecture, insisting on developing its own national system in parallel to the defenses operated by U.S. Forces Korea (USFK). American appeals for interoperability between U.S. and ROK systems have been received cautiously, as were proposals to enhance its own BMD in Korea by introducing the Terminal HighAltitude Area Defense (THAAD) to the Peninsula for several years. A desire for expanded autonomy in national security appears to underpin Seoul’s attitudes on BMD. Rather than rely passively on American protection against North Korea’s nuclear and missile threats, South Korea’s military leaders have focused on developing precision-strike capabilities to intimidate Pyongyang, and resisted simply accepting an American BMD umbrella. Even more than they desire greater independence from their American patron-ally, South Koreans are suspicious of entanglements with Japan, their former colonial master, whose own defensive systems are already integrated with the American regional BMD architecture. This outlook encourages the pursuit of independent defense capabilities and discourages institutionalizing trilateral security arrangements.