In the 1950s, during the Cold War, a strong interceptor force was crucial for the great powers as the best means to defend against an unexpected nuclear attack by strategic bombers. Hence, for a brief period of time they faced rapid development. At the end of the 1960s, a nuclear attack became unstoppable with the introduction of ballistic missiles capable of approaching from outside the atmosphere at speeds as high as 5–7 km/s. The doctrine of mutually assured destruction replaced the trend of defense strengthening, making interceptors less strategically logical. The utility of interceptors waned as the role merged with that of the heavy air superiority fighter, which was dominant in military thinking.