Over the past century, U.S. relations with Russia have evolved from ally to enemy to strategic partner to competitor. The political landscape and national interests of the Russian Federation have changed since the breakup of the Soviet Union. As a result, relations between Russia and the United States today are strained, largely because of Russia's actions in Ukraine. Understanding Russia's intentions have been challenging and difficult in the past for the United States. This monograph argues that Russia's foreign policy is driven by four overarching factors: Russian President Vladimir Putin's approach to the world around him; the Kremlin's desire for centralized control of the population; Russia's desire to protect its homeland through an outside "buffer zone;" and an enduring distrust of the West. Given these drivers of Russian foreign policy, deterring Russia without provoking conflict or creating a spiraling security dilemma is a difficult task. Russia's actions in Crimea and eastern Ukraine have put the Baltic States and Eastern Europe on edge. The primary challenge for the United States and the North Atlantic Treaty Organization (NATO) is to deter both a conventional threat and an ambiguous threat as Russia works toward achieving its objectives. The most dangerous scenario facing the West is a Russian advance into Alliance territory with conventional forces, but many assume this is not very likely. Alternatively, an indirect Russian approach using ambiguous warfare to fracture the Alliance and increase Russia's influence in Europe is far more likely. In attempting to devise solutions that would address both a conventional and an ambiguous threat, this monograph theorizes that based on current force structure, NATO lacks the capability to defeat a surprise Russian conventional attack into the Baltic States or Eastern Europe, regardless of the likelihood of such a scenario. However, this does not preclude the need to enhance conventional capabilities, modify force posture, and develop additional capabilities to counter both conventional and ambiguous threats, which will in turn underpin credible deterrence against Russian aggression. To develop such capabilities requires a concerted effort on the part of NATO, the European Union (EU), and their member states, with the United States playing a key role. Yet Washington cannot afford, through its efforts, to reassure allies to the point where they solely rely on the United States to ensure their security. Therefore, European NATO members should continue searching for more effective ways to increase capabilities and progressively increase their defense budgets. Meanwhile, the United States and its allies must employ a coordinated, whole of government effort to address capabilities beyond the scope of the military, such as law enforcement, that are critical to addressing an ambiguous threat. Additionally, the United States European Command (EUCOM) and the United States Army Europe (USAREUR) must more effectively align their security cooperation activities to support capability development, especially through NATO's defense planning process. In doing these things, the United States and NATO must be careful that reassurance and deterrence activities, and associated policies, do not provoke further Russian aggression, or lead to a new security dilemma. To that end, any policy or strategy toward Russia must understand Russian intentions and the likelihood of a conventional attack- balanced against the reality of potential ambiguous activities and Russian influence in Europe.