Edward III decided early in 1345 to attack France on three fronts. The Earl of Northampton would lead a small force to Brittany, a slightly larger force would proceed to Gascony under the command of Grosmont, and the main English army would accompany Edward to either northern France or Flanders.; ; Grosmont was appointed the King's Lieutenant in Gascony on 13 March 1345 and received a contract to raise a force of 2,000 men in England, and further troops in Gascony. The highly detailed contract of indenture had a term of six months from the opening of the campaign in Gascony, with an option for Edward to extend it for a further six months on the same terms. Derby was given a high degree of autonomy, for example, his strategic instructions were: "si guerre soit, et a faire le bien q'il poet" (...if there is war, do the best you can...).
