This dissertation examines the history of the relationship between the United States (US) and South Korea (Republic of Korea: ROK) from 1945 to 1975 in the broader context of the emergence, development, and transformation of nuclear dynamics. The term ‘nuclear dynamics’ used in this dissertation includes not only technological aspects of nuclear development for civilian and military purposes but also the application of nuclear technology to international and bilateral politics and its consequences. Thematically, nuclear dynamics are composed of three main pillars: nuclear energy, nonproliferation, and extended deterrence. Using this term, the dissertation questions how the nuclear dynamics have defined and affected the alliance politics between the United States and South Korea since the birth of nuclear weapons until the ROK signed the Treaty on the Nonproliferation of Nuclear Weapons (NPT) in 1975. To answer this question, the dissertation divides the thirty years of nuclear history from 1945 to 1975 into three sub-periods and seeks to connect each of them with the US-ROK relations. The first period goes from 1945 to the end of 1950s when Korea faced the newly developed nuclear weapons and technology for the first time and started to grasp the necessity of adjusting to the new era of nuclear power. The dissertation argues that in this era of emerging nuclear dynamics, Korea was an unintended beneficiary of the US-led development of nuclear technology; not only did the US atomic bombings on Hiroshima and Nagasaki in early August 1945 facilitate the liberation of Korea from Imperial Japan South Korea, which ultimately resulted in the formation of the US-ROK military alliance through the Korean War, but South Korea was also able to embark on a costly civilian nuclear program with US assistance under the Atoms for Peace program. On the other hand, however, the Korean Peninsula was turned into a military target during the Korean War as Washington considered bombing some parts of Korea and Manchuria with nuclear weapons to defeat Chinese and North Korean forces, an option which was dismissed by top US policy makers in the end. This dissertation thus argues that the role of South Korea was apparently limited in the early nuclear age because Korea remained as a passive beneficiary or recipient of nuclear technology as well as a potential target of atomic bombing; however, the Koreans started to become more involved in developing their own nuclear dynamics during the 1960s. The second period of the 1960s coincides with multiple symptoms of the bilateral relations going sour. Despite South Korea’s active participation in the Vietnam War to support the US military’s war efforts, the ROK government became less confident in the US security commitment to Korea in the late 1960s; Washington started considering pulling out its military from Vietnam and showed a lukewarm response to Seoul’s repeated requests to retaliate against Pyongyang for the North Koreans’ military provocations during this period. Under the circumstances, the widening gap between Washington and Seoul overlaps with the ROK’s increasing investment in civil nuclear research, including the construction of a nuclear power plant in a favorable environment for nuclear trade on international markets. This dissertation argues that the 1960s was a defining period for South Korea’s embryonic nuclear program, shaping the ambitions to develop nuclear weapons that would be pursued in the following decade. South Korea’s fear of abandonment by its patron, the United States, grew amidst the latter’s changing global and regional strategy in the late 1960s, and Korea’s interests in nuclear technology combined with its incipient capability in nuclear research provided an opportunity for embarking on a nuclear weapons program in the next period. The third period of US-ROK relations, which is characterized by the transformation of nuclear dynamics, began with newly inaugurated President Richard Nixon’s policy on disengaging from Korea and Vietnam. Nixon’s decision to withdraw US troops from Korea without previous consultation with Seoul further increased President Park’s anxiety over the fate of South Korea as well as his own regime. This dissertation argues that the nuclear weapons program entitled Project 890 was initiated in 1970 by President Park and his close aides as a way to develop an indigenous nuclear capability which would allow South Korea to become less dependent on US extended deterrence. At that time, Washington officially announced US troops reduction from South Korea. Although President Park was aware from the beginning that his military nuclear program would bring US pressure, he believed that the project would allow South Korea to acquire sufficient nuclear technology to make weapons later when deemed necessary. This research shows that South Korea’s passive position in nuclear diplomacy during the 1950s was drastically transformed into a more active one in the 1970s as Korea attempted to develop its own nuclear weapons. Under the guise of a civil nuclear project, the ROK government attempted to acquire sensitive nuclear technology and equipment necessary to pursue its military ambitions from other nuclear suppliers such as Canada, France, and Belgium, some of which were frustrated by Washington’s intervention. At the same time, the Koreans also made efforts to develop a delivery vehicle for nuclear weapons, which resulted in successfully launching a medium range of surface-to-surface missile in 1978. On the surface, South Korea’s pursuit of a military nuclear capability appeared to have been halted with its ratification of the NPT; however, history tells that it has been never completely ended until today. This dissertation concludes that the development of US-ROK bilateral relations has been closely intertwined with the emergence and change of nuclear dynamics in both civil and military arenas of nuclear development. In particular, the history of the thirty years from 1945 to 1975 shows a drastic change of South Korea’s status in the nuclear field as well as its relations with the United States. Moreover, it is hard to explain the ROK government’s current position towards issues related to nuclear energy and weapons without examining South Korea’s critical past, as the past experiences shaped Korea’s perspectives on the nuclear world. Thus, this work makes a contribution to showing the history of how South Korea adapted itself to a world with nuclear weapons in the post-war period.