"In 1958, Bomber Command's emergency war plan called for the destruction of 44 Soviet cities. Such an attack would kill about 38 million people. One hydrogen bomb would be dropped on the centre of each city, but Moscow would be hit by four and Leningrad by two. Had Britain gone to war alongside the US in the early 1960s, Bomber Command would have been asked to destroy an additional 25 Soviet cities. As air defences improved in the Soviet Union, the number of urban areas that Britain planned to destroy unilaterally was reduced. By the late 1960s, the missiles carried by Polaris submarines served as the British strategic deterrent, and they were aimed at fewer than a dozen Soviet cities. Until the end of the cold war, the complete destruction of the Soviet Union's capital – known as the "Moscow criterion" – was the UK's main objective."