The son and heir of Henry, 3rd Earl of Lancaster, and Maud Chaworth, Grosmont became one of King Edward III's most trusted captains in the early phases of the Hundred Years' War and distinguished himself with victory in the Battle of Auberoche.

An intelligent and reflective man, Grosmont taught himself to write and was the author of the book Livre de Seyntz Medicines, a highly personal devotional treatise.

As well as undergoing a number of periods of imprisonment as a debtor, Grosmont was eventually to raise numerous large sums, including £969 to gain the release of the royal crown, which Edward had pawned, and £1,500 worth of jewels to free the King after he had offered himself as security for loans.

[17] In 1328 Edward III's regents had agreed to the Treaty of Northampton with Robert Bruce, King of Scotland (r. 1306–1329), but this was widely resented in England and commonly known as turpis pax, "the cowards' peace".

In 1330 Edward III, who had recently assumed his full powers, made a formal request to the Scottish Crown to restore Beaumont's lands which was refused.

[23] The Scots felt compelled to attempt to relieve the siege and an army of 20,000 men attacked the English at the Battle of Halidon Hill, 2 miles (3.2 km) from Berwick.

Informed that the widow of the Earl of Atholl was besieged in Lochindorb Castle, Grosmont led a small force to rescue her and raise the siege, which he had achieved by 16 July.

Believing that he would soon be at war with France, Edward withdrew most of his forces from Scotland in mid-1336 and sent Grosmont to London to plan the defence of the English Channel ports from the mouth of the Thames westward.

This was a diplomatic mission amidst much pageantry; Edward III concluded agreements with a number of rulers, including Louis, whereby they would provide troops in exchange for payment.

[38] Edward III planned to invade France with his army of allies in 1339, but was finding it impossible to raise the money to pay them; he and his ambassadors had committed him to far greater expense than he could fund.

[47] Edward was unable to raise the necessary funds to repay his loans and on 25 September Grosmont returned to Malines to, as the King wrote in October, "lay in prison" again.

[49] On his return, he was made the king's lieutenant in the north, or Scotland, and stayed at Roxburgh until 1342 when a six-month truce allowed a number of jousts and tournaments to take place.

[53] 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 ...).

[54] On 9 August 1345 Grosmont arrived in Bordeaux with 500 men-at-arms, 1,500 English and Welsh archers – 500 of the latter mounted on ponies to increase their mobility[55] – ancillary and support troops.

[83] The garrison of Aiguillon, some 900 men, sortied repeatedly to interrupt the French operations, while Lancaster concentrated the main Anglo-Gascon force at La Réole, some 30 miles (48 km) away, as a threat.

[84] Among other consequences, this equivocation allowed Grosmont in the south-west to launch offensives into Quercy and the Bazadais; and himself lead a chevauchée 160 miles (260 km) north through Saintonge, Aunis and Poitou, capturing numerous towns, castles and smaller fortified places and storming the rich city of Poitiers.

Believing that Lancaster was heading for Paris, the French changed the assembly point for any men not already committed to Compiègne to Orléans, and reinforced them with some of those already mustered, to block this.

The two men were second cousins through their great-grandfather King Henry III and practically coeval (Edward was born in 1312), so it is natural to assume that a strong sense of camaraderie existed between them.

Another factor that might have influenced the King's decision was the fact that Henry had no male heir, so the grant of the dukedom was effectively made for Grosmont's lifetime only, and not expected to be hereditary.

[90] Heinrich von Herford reports that, soon after leaving the north, the Duke's vanguard was robbed by Lithuanian knights, hence his decision to return to England through Cologne.

This almost led to a duel between the two men in Paris, which was only averted by the personal intervention of the French king who, Fowler suggests, was himself unclear which one was the injured party—Grosmont for his original accusation of kidnapping, or Brunswick for its unchivalric intimation.

[92] While Grosmont's duel may have come to nothing, argues Fowler, more importantly, his lengthy sojourn abroad allowed him to do intelligence on the French court and assess its military or political divisions.

[93] Between then and 1355, Grosmont's time and energies were mainly spent acting in a diplomatic capacity to the Papal Curia following the death of Pope Clement in December 1352 and the complex election of his successor, Innocent VI, whose overarching strategy was peace between France and England.

[102] On 1 June 1356 an initial force of 140 men-at-arms, 200 archers and 1,400 horses left Southampton in 48 ships for the beaches near St. Vaast la Hogue in the north-east Cotentin.

[115] The historian Alfred Burne hypothesises that French siege equipment had been captured at Pont-Audemer and made storming fortified places a more viable proposition than earlier in the chevauchée, when they were avoided.

[115] The attack on Verneuil was probably motivated by the prospect of looting a rich town;[111] no attempt was made to relieve Navarrese-held Tillières-sur-Avre, 7 miles (11 km) to the east.

[119] The French at Condé-sur-Iton also rested, having marched hard to get there in two days from Rouen; John probably also wished for all of his stragglers and detachments to join his army before offering battle.

Lancaster replied ambiguously, but John, convinced that Grosmont's main reason for landing in Normandy was to seek a battle, believed an agreement to fight the next day had been reached and camped for the night.

[135] The Black Prince also returned towards his starting point, but his delay in waiting for Lancaster near Tours enabled a French army under the command of their king to overtake him.

[145] More is known about Lancaster's character than that of most of his contemporaries through his memoirs, the Livre de Seyntz Medicines ("Book of the Holy Doctors"), a highly personal treatise on matters of religion and piety, also containing details of historical interest.