On 14 February 1254 at Bazas, during his third and last visit to the duchy of Gascony, Henry III conferred upon his eldest son Edward the first ever apanage in English history. Composed mainly of the royal lands of Ireland, the county of Chester, the recent royal conquests in Wales, various towns and castles across the Midlands and on the Anglo-Welsh border, the Channel Islands, the duchy of Gascony and the Isle of Oléron, this apanage represented a considerable part of the original dominions directly controlled by the king.

While impressive, Henry III's grant to his son came about primarily as a result of the dramatic political situation in Gascony. Following the losses of Normandy, Anjou, Poitou, Touraine and Maine, Gascony was the last territory controlled by the English king in France. It was then of critical importance, not only symbolically, but economically, for the wine trade with England, and strategically, for the access and resources it could provide in the event of a military campaign on the Continent. Since the 1240s, however, the duchy had been problematic to administer. The insubordination of its lords, the strong tradition of local independence and self-governance, and the external pressure exerted by surrounding lordships, had exacerbated what was already a difficult situation. In 1242–3, Henry III had tried, for the first time in the history of the duchy, to reorganise and consolidate the English presence in the region. Despite encountering some success in stabilising Gascony, his efforts proved short-lived. In 1248 tension peaked again, and the king sought to restore order by appointing his brother-in-law, Simon de Montfort, as seneschal of Gascony. Unfortunately, Montfort's high-handed policy and harshness towards the local population only plunged the region into a state of civil war, and led to his replacement in November 1252.

After Montfort's removal by the king, the internal divisions in the duchy, and the absence of strong ducal leadership there, encouraged Alfonso X, newly crowned king of Castile, to press his claims over Gascony. The rights he held were relatively vague, but Alfonso supported the rebellion of local barons and certain Gascon nobles, such as Gaston de Béarn, recognised him as their legitimate lord. The situation rapidly threatened to spiral out of control, and worrying rumours of the loss of the duchy eventually compelled Henry III to act in forceful fashion.