Gaining support throughout Castile because of his relation to Alfonso XI and Peter's continuous military escapades, Henry built an alliance with Aragon and France, including mercenaries led by French constable Bertram Du Guesclin for another attempt at the Castilian crown in 1365. Peter gained the support of Edward the Black Prince, heir to the English throne and son of Edward III of England, to help defend his crown with the promise of territorial gains. On 13 April 1367, Peter and Edward's forces strongly defeated the armies of Francs, Aragonese, and Castilians led by Henry and captured Bertram Du Guesclin. As Edward fell ill, and sick with Peter's attempts to get Edward's prisoners executed, and perhaps with Peter's delay or failure to fulfill his promises of land to England, the Plantagenets withdrew from their direct battlefield support of the Castilian Crown to the new front in Gascony opened to the French. In March 1369, with the continued support of France and Aragon, and growing support in important cities in parts of Castile, Henry's forces again invaded the Castilian Crown's realm and checked Peter's army. Henry of Trastámara himself was responsible for the death of his brother, Peter I of Castile.