 Lancaster spent the <unk> intermittently campaigning and negotiating peace treaties with the French . In <unk> he was present at the naval victory at <unk> , 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 <unk> in Normandy in <unk> and the siege of Rennes in <unk> , Lancaster participated in the last great offensive of the first phase of the Hundred Years ' War : the <unk> campaign of <unk> @-@ 60 . Then he was appointed principal <unk> for the Treaty of <unk> , where the English achieved very favourable terms . 
