This project was developed within a PhD scholarship with the funding reference SFRH/BD/144062/2019.   During the Cold War, the United States (U.S.) perceived a constant threat of a Soviet nuclear attack. Threat perceptions of Moscow declined considerably in the U.S. as the end of the conflict approached, with both countries drastically reducing their nuclear arsenals through arms control agreements. However, in 2024, Washington and Moscow still retain arsenals totaling about 88% of the world’s nuclear weapons. Despite cooperation in the framework of the nuclear non-proliferation regime during the post-Cold War period, the U.S. and Russia maintain a relationship of nuclear deterrence, having abandoned or suspended arms control regimes, developed new nuclear weapons, and undertaken nuclear modernization programs. Advancing the hypothesis that threat perceptions of Russia have survived, this thesis questions how they explain changes in U.S. nuclear deterrence policy (2001-2021). Combining Securitization Theory with Political Discourse Analysis, we developed a methodology that views threat perceptions as part of a broader spectrum of reasons for action that inform conclusions about what should be done. Threat perceptions do not exist in a vacuum but coexist or compete with other reasons for action that, as a whole, explain specific policy choices. In this thesis, we argue that threat perceptions from Russia are ubiquitous reasons for action, often prevailing over others, and informing decisions to act, not to act, and how to act. The securitization of Russia in the U.S. during the period under analysis profoundly influenced decisions about missile defense, nuclear arms reductions, arms control, and nuclear modernization programs.  Keywords: United States, Russia, Nuclear weapons, Nuclear deterrence, Securitization spectrum, Political discourse analysis.