invaded France 28 Oct., and was brought to a conclusion by the treaty of Bretigny, 18 May 1360.
On the death of his father-in-law, March 1361, he succeeded, in right of his wife, to the earldom of Lancaster, and entered into possession of great estates, chiefly in the northern counties, which were confirmed by special charter. On 23 April he was created a knight of the Garter. Within a year he succeeded to the rest of the Lancastrian possessions by the death, on Palm Sunday, 10 April 1362, of Maud, the elder daughter of Henry of Lancaster and widow of William, duke of Bavaria; and at the same time took the titles of Earl of Derby, Lincoln, and Leicester. On 13 Nov. following he was advanced to the rank of Duke of Lancaster.
In 1364 Lancaster accompanied his brother, Edmund of Langley [q. v.], to Flanders, in order to negotiate a treaty of marriage between Edmund and Margaret, daughter of Count Louis. The contract was signed at Dover 19 Oct., but the match was broken off through French intrigue.
The expulsion of Pedro the Cruel from Castile by Henry of Trastamare in the early part of 1366 led to the first active interference of the English in the affairs of that country, which was destined to have so great an influence on the fortunes of John of Gaunt. Pedro took refuge at Bordeaux, and was welcomed by the Black Prince, who urged his father to support the dethroned king. Accordingly, Lancaster was despatched from England, and took part in the final arrangements with Pedro, September 1366. He then returned to England, where forces were being collected, and was ready to set out again for Guienne in command of them at the beginning of November. He did not, however, actually set sail until the beginning of the new year, 5 Jan. 1367. He landed in Brittany, and marched through Poitou and Saintogne to Bordeaux, and thence to Dax on the Adour, whither the Black Prince had already advanced with his army on the march to invade Spain. Lancaster was appointed captain of the vanguard, and led the first division of the army across the Pyrenees, through the pass of Roncesvalles, 20 Feb. 1367. The English force traversed the kingdom of Navarre, and, entering Castilian territory, occupied Salvatierra, and thence advanced towards Vittoria. During this march Tello, the brother of Henry of Trastamare, made an unexpected attack on Lancaster's camp in the early morning. The duke appears to have acted with presence of mind, drawing up his men in a good position to resist the enemy; but a detachment of his troops was destroyed almost to a man. The hostile armies lay in sight of each other for some days, when the Black Prince, straitened for provisions, suddenly retreated, and crossing the Ebro, took up a position under the walls of Logroño. Henry followed, and posted himself at Najera. On 2 April the English broke up their camp, and advanced to Navarete, and the next day the armies met between that place and Najera. The vanguard of the Castilians was led by Bertrand du Guesclin and the Marshal d'Audrehem, and was opposed by the division under Lancaster and Sir John Chandos. Froissart describes the duke as taking the lead in the first onslaught. The English were here victorious; Du Guesclin was taken prisoner; and Lancaster coming to the assistance of his brother in his struggle with the main body of the enemy, the battle was won. The victory of Najera restored Pedro to his throne, but brought no advantage to the English. They occupied Burgos for some three weeks, and then went into quarters at Valladolid, awaiting the fulfilment of Pedro's engagements. He, however, showed no readiness to discharge his debts, sickness broke out, and the mortality was so great that scarcely a fifth of the army is said to have survived. The Black Prince himself was stricken; Henry, who had escaped into France, was threatening Aquitaine; and a speedy retreat from Spain became imperative. This was safely effected, and the prince and Lancaster reached Bordeaux early in September, Lancaster returning thence to England.
The bad faith of Don Pedro towards his English allies, the consequent license of the unpaid free companies, and the levy of unpopular taxes conspired to arouse the hostility of the people of Guienne against the English occupation. Charles of France profited by this discontent, and during the next year made preparations for a rupture of the treaty of Bretigny. On 20 March 1369 he declared war, marched straightway into Ponthieu, and conquered it. Preparations had, however, already been commenced in England for sending reinforcements into the English dominions in France. On 12 June Lancaster was appointed captain and lieutenant of Calais and Guines, and on the arrival of news that the French king was gathering troops for the invasion of England, he was despatched to Calais early in August in command of a body of six hundred men-at-arms and fifteen hundred archers. But no result followed. After some raids in the neighbourhood, the English drew out between Ardres and Guines, where they were joined by Robert of Namur with reinforcements.