Henry of Grosmont, Duke of Lancaster (– 23 March 1361) was an English statesman, diplomat, soldier, and Christian writer. The owner of Bolingbroke Castle in Lincolnshire, Grosmont was a member of the House of Plantagenet, which was ruling over England at that time. He was the wealthiest and most powerful peer of the realm.

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. He was a founding member and the second knight of the Order of the Garter in 1348, and in 1351 was created Duke of Lancaster. 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. He is remembered as one of the founders and early patrons of Corpus Christi College, Cambridge, which was established by two guilds of the town in 1352.

Origins Henry of Grosmont was the only son of Henry, 3rd Earl of Lancaster (–1345), who in turn was the younger brother and heir of Thomas, 2nd Earl of Lancaster (–1322). They were sons of Edmund Crouchback, 1st Earl of Lancaster (1245–1296), the second son of King Henry III (ruled 1216–1272) and younger brother of King Edward I of England (ruled 1272–1307). Henry of Grosmont was thus a first cousin once removed of King Edward II and a second cousin of King Edward III (ruled 1327–1377). His mother was Maud de Chaworth (1282–1322). On his paternal grandmother's side, Henry of Grosmont was also the great-great-grandson of Louis VIII of France.

Little is known of Grosmont's childhood and youth. Due to his uncle Thomas' increasingly hostile relations with King Edward II—culminating in his rebellion and execution in 1322—historian Kenneth Fowler suggests that this period "must have been quite eventful", and that Grosmount's "fortunes and opportunities were inextricably bound up with the tragic events of these years". The year and place of his birth are not known with certainty. He is believed to have been born at Grosmont Castle in Grosmont, Monmouthshire, Wales. According to his own memoirs he was a "good looking, youth. Tall fair and slim", and better at martial arts than at academic subjects; he did not learn to read until later in life. His education would have been orthodox, but in being so, produced the archetypal young noble admired at the time.

Henry of Grosmont was the eventual heir of his wealthy uncle Thomas, 2nd Earl of Lancaster, who through his marriage to Alice de Lacy, daughter and heiress of Henry de Lacy, 3rd Earl of Lincoln, had become the wealthiest peer in England. Constant quarrels between Thomas and his first cousin, King Edward II of England, led to his execution in 1322. Having no progeny, Thomas's possessions and titles went to his younger brother Henry, 3rd Earl of Lancaster, Grosmont's father. Henry of Lancaster assented to the deposition of Edward II in 1327, but fell out of favour with the regency of his widow Queen Isabella and Roger Mortimer. When Edward III, the son of Edward II, took personal control of the government in 1330, relations with the Crown improved, but by this time Henry of Lancaster was struggling with blindness and poor health. He was able to attend the new king's coronation on 1 February 1327, where he knighted Edward. Grosmont was knighted the same year, represented his father in Parliament and attended the King's council. He is known to have travelled a great deal between his father's estates, presumably supervising their management. In 1331 he participated in a royal tournament at Cheapside in the City of London. It was, argues Fowler, conducive to Lancastrian relations with the crown that the new King and Grosmont were of a similar age and interests, and Grosmont was never to experience the reversals of fortune his father had faced.

Estates and finances Originally heir to only the lordships of Beaufort and Monmouth, by 1327 it was looking probable that he would inherit most of the Lancastrian patrimony. He was not yet independent from his father, from whom he received regular fundsamounting to £666 in 1332as well as grants from the English crown as a reward for his service and in recognition of his lack of a personal income. From early the next year, his father began transferring his South Wales estates into Grosmont's name, giving him an independent income. In March 1337 he was one of six men Edward III promoted to higher levels of the peerage; one of his father's lesser titles, the Earldom of Derby, was bestowed upon him. He was also granted a royal annuity of 1,000 marks (£ as of ) for so long as his father lived, and a number of lucrative estates and perquisites were settled on him. By this point Grosmont's future relationship with the crown was "actively assured" comments Fowler.

The early 1340s were a period of financial constraint for Grosmont due to his acting as a broker for a number of loans between the King and Flemish bankers. 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.

Scotland In 1328 Edward III's regents had agreed to the Treaty of Northampton with Robert Bruce, King of Scotland (), but this was widely resented in England and commonly known as turpis pax, "the cowards' peace". Some Scots nobles refused to swear fealty to Bruce and were disinherited. They left Scotland to join forces with Edward Balliol, son of King John Balliol (), whom Edward I had deposed in 1296. One of these was Grosmont's father-in-law, Henry de Beaumont, Earl of Buchan and veteran campaigner of the First War of Scottish Independence. Robert Bruce died in 1329; his heir was 5-year-old David II (). 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. In 1331 the disinherited Scottish nobles gathered in Yorkshire, and led by Balliol and Beaumont plotted an invasion of Scotland. Edward III was aware of the scheme but turned a blind eye. Balliol's forces sailed for Scotland on 31 July 1332. Five days after landing in Fife, Balliol's force of some 2,000 men met the Scottish army of 12,000–15,000 men and crushed them at the Battle of Dupplin Moor. Balliol was crowned king of Scotland at Scone on 24 September 1332. Balliol's support within Scotland was limited and within six months it had collapsed. He was ambushed by supporters of David II at the Battle of Annan a few months after his coronation and fled to England half-dressed and riding bareback. He appealed to Edward III for assistance.;

On 10 March Balliol, the disinherited Scottish lords and some English magnates crossed the border and laid siege to the Scottish town of Berwick-upon-Tweed. Six weeks later a large English army under Edward III joined them, bringing the total number of besiegers to nearly 10,000. Grosmont was present at the Siege of Berwick (1333), but it is not known if he marched with his father-in-law and Balliol, or with the main English effort. 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, from Berwick. Under intense bow-fire the Scottish army broke, the camp followers made off with the horses and the fugitives were pursued by the mounted English knights. Scottish casualties were numbered in thousands, including their commander and five earls dead on the field. Scots who surrendered were killed on Edward's orders and some drowned as they fled into the sea. English casualties were reported as fourteen; some chronicles give a lower figure of seven.; About a hundred Scots who had been taken prisoner were beheaded the next morning, 20July. It is presumed that Grosmont took part in the battle, but it is possible that he was part of the detachment posted to ensure that the garrison of Berwick did not sally. Berwick surrendered the day after the battle and Grosmont witnessed and sealed the articles of surrender and, a little later, the town's new charter.

Balliol having sworn fealty to David II, and because he intended to make Scotland subservient to the English, most Scots refused to accept Balliol as their monarch. In December 1334 Grosmont accompanied Edward III to Roxburgh in Scotland. The English force of 4,000 accomplished little and withdrew in February. Grosmont was a member of Edward III's negotiation team when a brief truce was agreed shortly after at Nottingham. In July Grosmont accompanied Edward III on another invasion of Scotland, with an army of 13,000for the time an extremely large force. Scotland was quelled as far north as Perth and Grosmont took a senior role in raiding deeper into the country. In 1336 Grosmont was given command of 500 men-at-arms and 1,000 longbowmen and marched to Perth. 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. He was given full plenipotentiary powers by the King. After Edward III reached Perth with the main English army, Grosmont was despatched on a long-range raid to Aberdeen, away. He returned after two weeks, having razed Aberdeen and devastated the country on the way. Edward went south for six weeks, leaving Grosmont in charge of English-occupied Scotland. 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. By May the following year he was once again in Scotland—accompanied by the Earls of Warwick and of Arundel—and by the time he returned to London, war with France had commenced. Fowler suggests that "while in all these military enterprises Henry seldom played a leading role, they formed an invaluable apprenticeship for the years ahead".

Northeast France

Grosmont's first trip abroad was probably to Brabant in 1332. Six years later he travelled with Edward III to Flanders, which began inauspiciously when the King was "unable to pay or induce" his erstwhile allies to attack the French. Grosmont attended Edward's ostentatious meeting with the Holy Roman Emperor, Louis IV, at Coblenz. 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. Edward was appointed Imperial vicar. To cement the Flemish alliance, and on account of Edward's shortage of money, Grosmont and the Earl of Salisbury were imprisoned as sureties for Edward providing the balance of payment due his allies. Throughout the year French naval forces ravaged the English south coast. 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. Grosmont led part of Edward III's army when it finally invaded France in September 1339. Cambrai was besieged, the area around it devastated, and an unsuccessful attempt made to storm the town. The allied army pressed further into France, but the French refused to battle. In mid-October, the French issued a formal challenge to battle. Edward accepted and occupied a strong defensive position at La Capelle which the French declined to attack; Grosmont commanded a detachment. Having run out of provisions, money and weather suitable for campaigning, the allied army withdrew and dispersed. Grosmont arrived in Brussels with the army at the end of October, where the campaign was celebrated with a tournament.