The United States makes significant investments in military activities that are intended to deter Russian and Iranian aggression. These investments have only grown in Europe since 2014, when Russia invaded and subsequently annexed Crimea, and remain substantial in the Middle East despite the overall trend of the United States reducing its forward posture in that theater. The increased importance of deterrence as a military mission raises the question of how the United States can most effectively and efficiently deter Russia and Iran without crowding out investments in its other key military missions--including competing with China in the Indo-Pacific. To support defense planners in crafting effective and efficient deterrence strategies, RAND researchers conducted a multimethod analysis--consisting of a literature review, roundtables with subject-matter experts, quantitative analysis, and a case study of Ukraine--to examine conventional deterrence in two theaters: U.S. European Command (EUCOM) and U.S. Central Command (CENTCOM). Specifically, the researchers assessed the deterrent impacts of three categories of U.S. operations, activities, and investments (OAIs): U.S. forward presence; exercises and short-term deployments, such as bomber task force (BTF) missions; and security cooperation. In this report, the researchers describe their findings and offer recommendations for defense planners. This research was completed before the February 2022 Russian invasion of Ukraine. It has not been subsequently revised.