The most common reason for forming an alliance is a common security threat among the alliance partners. In the early years of the South Korea–United States alliance, the chief concern was the threat of communist expansion through the joint action of Moscow, Beijing, and Pyongyang. As the years passed, it became clear that the Soviet Union and China were not interested in starting another war in Korea. With the end of the Cold War, both former enemies established normal diplomatic relations with South Korea, much to the dismay of North Korea. Threat assessments focused solely on North Korea, but by the 2000s these assessments began to diverge among leaders in Seoul and Washington, leading to serious friction within the alliance. This chapter examines the security concerns that underlie the alliance and how the assessment of these concerns has evolved over time.