King Edward aided his son by raising an army in England, which sailed for Bordeaux under the command of the prince's brother, John of Gaunt, Duke of Lancaster. Walter formed part of this expedition. The king had issued his writs to him and other barons of the southern counties, and the Black Prince had himself written to ask him to join him, in memory of their former deeds of arms together.

As it was now some years since he had taken the field, Walter did not hesitate, but with thirty retainers, headed by Ralph, joined the army of John of Gaunt.

The Black Prince's first step was to endeavour to recall the Englishmen of the free companies, estimated to amount to at least 30,000 men. The news that he was taking up arms and would himself command the army caused Calverley and the whole of the other English knights to return at once, and 10,000 of the English men- at-arms with the great companies also left Don Henry and marched to Aquitaine. The road led through the territory of the King of Navarre, and the Black Prince advanced 56,000 florins of gold to pay this grasping and treacherous king for the right of passage of the army.