<p>This article explains why the United States–South Korea (US–ROK) alliance endured during Donald Trump’s first presidency, despite unprecedented abandonment threats and widespread predictions of rupture. Drawing on the autonomy-security trade-off (AST) model, it argues that Trump’s coercive bargaining re-activated the alliance’s foundational asymmetry by compelling Seoul to yield partial autonomy across three domains: increased cost-sharing, deferral of wartime operational control (OPCON) transfer, and expanded procurement of US weapons. These outcomes confirm AST’s core prediction that asymmetric alliances persist when weaker states trade sovereignty for protection. Yet the South Korean case also demonstrates that such concessions are conditional: they were strategically framed, temporally deferred, or symbolically performed in order to mitigate nationalist backlash and preserve domestic legitimacy. Methodologically, the article treats cost-sharing negotiations as a least-likely test of AST, showing that even extreme abandonment threats produced measurable compliance rather than rupture. Theoretically, it refines AST by incorporating the mediating role of democratic audience costs, elite discourse, and sovereignty narratives in shaping how autonomy concessions are made. The findings suggest that alliance resilience under coercion rests less on institutional trust or shared norms than on adaptive sovereignty bargains negotiated under external pressure and internal political constraint.</p>