The US–Republic of Korea (ROK or South Korea) alliance underwent severe strains during the Trump era. However, we can only understand such strains by situating them in a larger post-Cold War context, wherein successive US and ROK administrations have sought to transform and upgrade the alliance. The Trump administration’s unorthodox policy approach accelerated alliance tensions and thus revealed the psychological and institutional limits and contradictions inherent to this larger transformation, even if Trump did not create the underlying dynamics driving them. This chapter explores these tensions through a survey of several issue areas under Trump in relation to which the allies held contrasting if not clashing positions, including perceptions of the North Korean threat and engagement with Pyongyang; the transfer of wartime operational control (OPCON) of the ROK military from the US back to South Korea; alliance cost-sharing; and the alliance’s place within the wider region. While alliance relations have returned to a degree of normalcy under President Biden, these same issues that caused alliance discord under Trump are no less salient today.