Lancaster spent the 1350s intermittently campaigning and negotiating peace treaties with the French . In 1350 he was present at the naval victory at Winchelsea , where he allegedly saved the lives of the Black Prince and John of Gaunt . The years 1351 @-@ 2 he spent on crusade in Prussia . It was here that a quarrel with Otto , Duke of Brunswick , almost led to a duel between the two men , narrowly averted by the intervention of the French king , John II . In the later half of the decade campaigning in France resumed . After a chevauchée in Normandy in 1356 and the siege of Rennes in 1358 , Lancaster participated in the last great offensive of the first phase of the Hundred Years ' War : the Rheims campaign of 1359 @-@ 60 . Then he was appointed principal negotiator for the Treaty of Brétigny , where the English achieved very favourable terms .