July 5, 1943 saw the costliest day in the history of aerial warfare and the opening of the deadliest armoured battle ever fought. This episode explains why the 160-mile Kursk salient invited Field Marshal von Manstein's Operation Citadel, a double envelopment meant to restore German prestige and capture hundreds of thousands of prisoners as slave labour. The Soviets already knew the plan: the Lucy spy ring in Switzerland passed the details up the chain, and Marshal Zhukov persuaded a newly receptive Stalin to abandon his instinct to strike first and instead build a defence in depth.

Hitler delayed for two months to field Panthers, Ferdinands and more Tigers, despite warnings from Model and Guderian and his own admission that the offensive made his stomach turn. The Soviets used that time to dig six defensive belts up to 150 miles deep, lay  ...