 Complete the following phrase:  Ralph, Earl of Stafford, had sailed for Gascony in February 1345 with an advance force and, following conventional practice, laid siege to two French strongholds. Derby arrived in August and immediately concentrated available Anglo-Gascon forces and headed directly for the largest French force, which was gathering at Bergerac, east of Bordeaux. Bergerac had good river supply links to Bordeaux and would provide a suitable forward base from which to carry the war to the French. He decisively defeated the French there, before moving to besiege the provincial capital of Périgueux. By this time the French had diverted their main effort to the south-west, under the overall command of John, Duke of Normandy, the son and heir apparent of King Philip VI of France. Unable to take Périgueux, and threatened by Johns much larger force, Derby left garrisons blockading it and withdrew. One garrison, at Auberoche, was besieged by the French. Derby advanced with a small force, launched a 