For the first 25 years after the fall of the Berlin Wall, the United States and its allies in the North Atlantic Treaty Organization (NATO) planned, postured, and engaged in Europe on the assumption that Russia was militarily capable of harming NATO and its European partners but did not intend to do so. Despite occasional tensions in relations between Washington and Moscow, Russia was widely assumed to be on a trajectory toward closer integration and more-peaceful relations with Europe, the United States, and its other neighbors. The Kremlins 2014 annexation of the eastern Ukrainian region of Crimea and active efforts to occupy and destabilize other parts of eastern Ukraine sharply challenged this underlying assumption. Aggression in Ukraine, combined with Russian snap exercises on NATOs borders, multiple aerial incursions into NATO and partner territory, cruise missile modernization, dangerous nuclear blustering, anti-Western rhetoric, and domestic political uncertainty, have forced a deep reassessment of U.S. strategy, plans, and posture in Europe and other regions in which Russia is active. The Kremlins intervention on behalf of the Bashar Al-Assad regime in Syria underscored its willingness to use force for a broad range of objectives that run counter to those of the United States.