Hans Kristensen is the director of the Federation of American Scientists' nuclear information project and the co-author of a recent article that argued the killing power of the US's ballistic missile arsenal has increased by a factor of three due to its modernisation programme. Modern weapons are being designed for tactical nuclear war - a development that has contributed to the Doomsday Clock being moved to two and a half minutes to midnight - the closest it's been since the height of the cold war in the 1950s.