In the 1930s, a group of American airmen had a dream. Air power, they believed, would do away with the need for armies and navies. The aeroplanes of the future would be able to drop bombs so accurately that there would be no need to kill soldiers in their millions: a handful of strikes on a few key factories would be enough to cripple an enemy’s economy and force them to sue for peace. It did not take long for this dream to turn sour. When the second world war broke out, the Americans soon discovered that their precision bombing was not nearly so precise when they were being shot at. When they flew high, clouds often obscured their targets. When they flew low, they were so vulnerable that they had to hide under cover of night — which also meant they could not see their targets.