Since America dropped the bomb on Hiroshima and Nagasaki in 1945, a fragile balance of deterrence, treaties, fear and taboo has stopped the world’s nuclear powers from deploying their arsenals in anger. But Russia’s invasion of Ukraine has ushered in a new nuclear era. How should we think about the nuclear threat? And what role should America play in policing it?   Dr Nina Tannenwald, author of “The Nuclear Taboo”, explains how the norms that guaranteed the long nuclear peace have been unravelling for years. We explore the lost era of US-Soviet collaboration to contain the threat from “loose nukes”. And Dr Siegfried Hecker, who led those efforts as director of the Los Alamos National Laboratory, shares lessons from sixty years working to avert nuclear catastrophe. John Prideaux hosts with Shashank Joshi and Jon Fasman For full access to print, digital and audio editions, as well as exclusive live events, subscribe to The Economist at economist.com/uspod. Subscribers can also sign up to our “Checks and Balance” newsletter at economist.com/newsletters.   See acast.com/privacy for privacy and opt-out information.