The risk of nuclear war casts a dark shadow over the already-tragic events in Ukraine. On February 19, Vladimir Putin launched an exercise involving Russian strategic forces. On February 24—the day Putin invaded Ukraine—he warned that Russia would respond immediately to those who stood in its way, with consequences that “will be such as you have never seen in your entire history.” On February 27, he publicly ordered his minister of defense and chief of the general staff to transfer Russia’s “deterrence forces” to “special combat readiness.” Putin’s aim was evidently to deter outside intervention and to signal Russia’s determination to achieve its goals.

But another, more troubling, aspect to Putin’s recent comments has received little or no attention. It has to do with the circumstances under which Russia might use nuclear weapons.