Russias February 2022 invasion of Ukraine triggered the bloodiest war in Europe since World War II and raised significant questions about the United States role in Europe. This CSIS report examines the U.S. force posture in Europe—including the military capabilities, personnel, infrastructure, and agreements that support defense operations and plans—and makes recommendations for future U.S. posture. It finds that the United States needs a robust, long-term military force posture in Europe, focused on the North Atlantic Treaty Organizations (NATO) eastern flank, to deter future Russian aggression.