It is time to debate the relevance of the concept of deterrence in the 21st century, when non-military threats such as climate change, pandemics, cyber, terror and “active measures” may prove more formidable dangers than opposing armies, navies and air forces. Deterrence emerged as a central and simple concept in the Cold War and the nuclear age. The argument was that if the two superpowers maintained enough nuclear capability to destroy the other after being attacked first, war would be suicidal and hence deterred. MAD, for Mutual Assured Destruction , became the operational acronym. Indeed, mutual nuclear annihilation does seem very mad. Two scorpions in a bottle, stinging each other to death, was the image of the MAD age. But suppose one were male; the other female. That possibility was ignored.