Abstract Whatever the outcome of the Russo-Ukrainian War, in its wake Ukraine will need to choose a security policy to defend its sovereignty from future threats. Its choice holds implications for broader European security. Some observers advocate Ukraine becoming a member in the North Atlantic Treaty Organization (NATO), thereby gaining protection from the U.S. “nuclear umbrella.” Others doubt the effectiveness of “extended nuclear deterrence”—the threat of U.S. nuclear retaliation for attacks, including those carried out with conventional armed forces, on an ally's territory. But nuclear deterrence was never put to the test in Cold War Europe, and today extended nuclear deterrence is an unreliable and risky approach to Russian aggression. An examination of the role of nuclear deterrence during the 1961 Berlin Crisis demonstrates that Soviet military strategy against U.S. nuclear weapons posed the risk of escalation. In vulnerable NATO territories, such as the Estonian city of Narva, such a risk still exists. A Cold War–era alternative to nuclear deterrence offers the possibility of a non-nuclear defense for Ukraine. Proposals such as the “spider in the web” strategy draw on concepts of the security dilemma and non-offensive, confidence-building defense to provide for Ukrainian security in a Europe threatened by Russian expansion, without relying on the threat of nuclear war.