Spain was the strongest naval power of the western world, and not only would the ports of Guienne be closed but its communication with England would be at once cut off by the appearance of a joint French and Spanish fleet in the Channel.

It was with satisfaction therefore that Edward saw the growth of a bitter hostility between Charles and the Castilian king, Pedro the Cruel, through the murder of his wife, Blanche of Bourbon, the French king's sister-in-law.

Henry of Trastamara, a bastard son of Pedro's father Alfonso the Eleventh, had long been a refugee at the French court, and soon after the treaty of Bretigny Charles in his desire to revenge this murder on Pedro gave Henry aid in an attempt on the Castilian throne.

It was impossible for England to look on with indifference while a dependant of the French king became master of Castille; and in 1362 a treaty offensive and defensive was concluded between Pedro and Edward the Third.

The time was not come for open war; but the subtle policy of Charles saw in this strife across the Pyrenees an opportunity both of detaching Castille from the English cause and of ridding himself of the Free Companies.