In 1345 Henry of Lancaster, earl of Derby - the most prominent soldier, diplomat and statesman of his generation - led an English royal army to the duchy of Aquitaine and inflicted two devastating defeats on the French royal forces. These were the first decisive victories for either side, and swung the course of the Hundred Years' War dramatically in England's favour. The remarkable success of the expedition, however, has been overshadowed in history by Edward III's more celebrated victory at Crecy the following year. This reassessment of a neglected campaign draws on a wealth of original...