Prince Edward's brother John of Gaunt, 1st Duke of Lancaster assumed leadership of the English in France. Despite further chevauchées, destroying the countryside and the productivity of the land, his efforts were strategically ineffective. The French commander, Bertrand Du Guesclin adopted Fabian tactics in avoiding major English field forces while capturing towns, including Poitiers and Bergerac. In a further strategic blow, English dominance at sea was reversed by the disastrous defeat at the Battle of La Rochelle, undermining English seaborne trade and allowing Gascony to be threatened.