On the afternoon of October 6, 1973, on Yom Kippur, the holiest day in the Jewish calendar, the armies of Egypt and Syria launched major assaults against Israeli positions along the Suez Canal and in the Golan Heights. It has long been rumored that in this desperate context Israel alerted or somehow manipulated its nuclear forces – perhaps in order to “blackmail” the United States into providing greater support, as one American journalist alleges, or to deter further Arab assault. If true, this would constitute one of the very few serious nuclear “threats” of the nuclear era. This in and of itself makes it a topic of enduring interest. But in light of the continued and perhaps growing salience of nuclear weapons – and thus also their political “uses”– in the hands of U.S. adversaries as well as allies and partners, this study is of more than antiquarian interest because, in concert with other examples drawn from crises and conflicts, it helps elucidate how nuclear weapons can affect and influence the course of politics and war. This study is the first of this kind on this incident and represents the results of almost a year of extensive research in U.S. Government archives and in the open literature, numerous interviews with participants and experts, and the convocation of a workshop to discuss the issue.