This chapter explores how US allies participate in US-led sanctions, focusing on South Korea’s response to Iranian sanctions following the Comprehensive Iran Sanctions, Accountability, and Divestment Act of 2010 (CISADA). South Korea faced an acute dilemma between supporting the US, its strongest ally, and maintaining its valuable economic and diplomatic ties with Iran. While other US allies swiftly implemented their own independent sanctions following the US lead, South Korea ultimately developed a distinctive approach, creating a complex sanctions regime across finance, trade, transportation, and energy sectors, while simultaneously establishing a Korean won-based settlement system for commercial transactions with Iranian parties. This strategy proved highly successful, allowing South Korea to balance competing international pressures without jeopardizing its relationships with either the US or Iran. South Korea’s case highlights an underexplored dimension of sanctions alliances: how countries may reluctantly join sanctions alliances, driven more by the need to avoid alienating a powerful ally than by a genuine commitment to the sanctioning campaign. This chapter provides new insights into the hidden dynamics of sanctions alliances and the sophisticated diplomatic strategies employed by nations navigating complex international obligations.