America and its allies are entering a period of nuclear uncertainty

IS DETERRENCE easy or hard? That simple question has been at the heart of nuclear strategy for almost 80 years. For Bernard Brodie, an early nuclear theorist, the bomb had created a stable balance of terror. The precise number and variety of weapons was less important than the fact that they existed. His colleagues Herman Kahn and Albert Wohlstetter disagreed. The balance was “precarious”, they retorted, and required careful and continuous attention to metrics such as the relative damage that each side would suffer in a nuclear exchange and thus to the relative size and quality of their respective arsenals.In America that debate is returning to the fore. A growing number of influential American thinkers believe that deterrence is becoming harder and could become precarious. Great-power rivalry has grown, making an intensification of nuclear competition more likely. Arms-control agreements have frayed. Amid the rising tensions, which have helped bring renewed attention to the , the spectre of Donald Trump looms large. His return to office could see a disruption to American alliances that prompts allies in Asia and Europe to look afresh at their own nuclear options.