Regional security analyses of Northeast Asia focus largely on North Korea, China,and the ongoing maritime disputes within the region. There is less discussion,however, on the wider security outlook of the region and its impact on the individual countries of Northeast Asia. This paper analyzes the security environment of Northeast Asia and how it affects the defense decision-making of the Republic of Korea. This paper also assesses the current state of the South Korean alliance with the United States and considers how the state of this alliance affects Korean security decision-making, and finds that South Korea is attempting to balance the regional security threats it perceives with changing domestic political demands. The security developments South Korea is pursuing may lead to deterioration in regional security, but the United States may be able to support regional stability through its alliances and forward presence. In analyzing both external and internal factors that influence South Korean defense decision-making, this research helps to contextualize these decisions and inform readers of the complex environment in which those decisions occur.