 In 1345 Edward III was planning a major assault on France . A three @-@ pronged attack would have the Earl of Northampton attacking from Brittany , the king himself from Flanders , while Grosmont was dispatched to Aquitaine to prepare a campaign in the south . Moving rapidly through the country , he confronted the Comte d ’ Isle at Auberoche on 21 October and there achieved a victory described as " the greatest single achievement of Lancaster 's entire military career " . The ransom from the prisoners has been estimated at £ 50 @,@ 000 . The next year , while Edward was carrying out his Crécy campaign , Grosmont laid siege to , and captured , Poitiers , before returning home to England in 1347 . 
