Assuring Allies and Partners: The Utility of Ballistic Missile Defense

The contemporary multipolar environment heightens threats from nuclear‑armed states such as Russia, China, and North Korea, raising doubts about US resolve in extended deterrence. Legal and policy constraints prevent Washington from transferring nuclear weapons and require reducing their role in strategy, even as the nuclear umbrella supports nonproliferation while complicating negative security assurances. This study evaluates ballistic missile defense (BMD) as a non‑nuclear assurance tool through case studies of the Republic of Korea, Japan, and Poland. Examining foreign military sales, technology‑sharing, and each state’s strategic and domestic context, it finds that BMD can contribute to assurance but only under specific conditions: trust in the United States, threat perception, preferred defensive strategy, and US willingness to tailor systems to partner needs. BMD cannot substitute for offensive capabilities; sustaining credible extended deterrence remains essential to prevent allies from pursuing nuclear hosting or indigenous nuclear options.