Henry, Lord Beaumont was a French nobleman who played an important role in Edward II's reign and in the early years of Edward III's.
Henry was born around 1280 as the elder son of Louis de Brienne, sometimes called Louis of Acre (c. 1225-c. 1297) and Agnes de Beaumont, vicountess of Beaumont-au-Maine (died 1301). He and some of his siblings took their mother's name. Henry was the nephew of, among others, Yolande, queen of Jerusalem in her own right, John de Brienne, stepfather of Alexander III of Scotland, and Marie de Brienne, married to the titular emperor of Constantinople. Through his paternal grandmother Berenguela of Leon, Henry was, like Edward II, a great-grandson of Queen Berenguela of Castile and King Alfonso IX 'the Slobberer' of Leon, which makes him Edward's second cousin. Henry and his siblings are often described as cousins of Isabella of France, though in fact they were more closely related to Edward II than to his queen.
Henry's sister Isabella, who must have been many years his senior - their parents had married as early as 1253 - arrived in England probably in the late 1270s, and married John, Lord Vescy (1244-1289) in 1280. Henry and his younger brother Louis, a cleric, followed their sister to England in the 1290s, and would spend the rest of their lives there, high in favour with Edward I and Edward II. Henry was a knight of Edward I's household in 1297, and the king granted him 200 marks a year in 1301, though it was after the accession of Edward II in 1307 that he reached the heights of royal favour. Edward arranged his marriage to Alice Comyn, one of the two nieces and co-heirs of John Comyn, earl of Buchan (d. 1308), in or shortly before 1310, which gave Henry a strong claim to the earldom.
Edward appointed Henry joint warden of Scotland in 1308, and he was first summoned to parliament in 1309. His closeness to the king made him a target of the Lords Ordainer, who ordered in late 1311 that he and his sister Isabella Vescy be removed from court. Edward had in 1308 granted Henry the Isle of Man, and the Ordainers demanded instead that it be given to "a good Englishman." Henry ignored their demands to stay away from the king, and was with Edward in Yorkshire in early 1312 when Piers Gaveston returned from his third exile. For many years, he was a staunch supporter of the king, fighting for Edward at Bannockburn and attending Piers Gaveston's funeral in January 1315. In early September 1314, Edward empowered Henry, with the earl of Pembroke and the bishop of Exeter, to open the York parliament on his behalf, claiming that he had urgent business elsewhere. (In fact, he merely took himself off to the small village of Oulston for a few days, almost certainly a doomed attempt to avoid his enemies in parliament after his humiliating defeat in Scotland.)
Henry was instrumental in the 1317 election of his brother Louis as bishop of Durham, after telling Edward II that if Louis "or another person of noble origin had the rule of the church of Durham a defence like a stone wall [against the Scots] would be provided for those parts," and persuading Edward to promote Louis's candidacy with the pope. Louis was supposedly illiterate, and had two club feet: the Lanercost chronicler calls him "a Frenchman of noble birth, but lame on both feet, nevertheless liberal and agreeable." Henry and Louis were involved in a somewhat mysterious affair in September 1317, when they were attacked, robbed and taken prisoner by Sir Gilbert Middleton, while travelling to Durham in the company of two cardinals for Louis's consecration. This attack may have been the work of the earl of Lancaster, annoyed at the failure of his own candidate to gain the bishopric, though an indignant Pope John XXII blamed the Scots.
Henry remained staunchly loyal to Edward during his 1321/22 campaign against the Marcher lords, and fought against the earl of Lancaster at the battle of Boroughbridge on 16 March 1322. He also accompanied the king on the Scottish campaign later that year, and it was Edward's thirteen-year truce with Scotland in May 1323 which finally pushed Henry into opposition to him. Henry had a strong claim to the earldom of Buchan via his wife, and if Edward made peace with Scotland, Henry would never be able to claim his title and lands. At a meeting of Edward's counsellors at Bishopthorpe on 30 May 1323, Edward asked their advice about the truce. Henry, "with an excessive motion and irreverent mind," refused to advise the king, and continued to refuse. Edward lost his temper and ordered him out of the room, whereupon Henry retorted that "it would please him more to be absent than to be present." Five days later, Edward ordered his arrest for this "contempt and disobedience." 
Henry was probably also angry at a bitterly sarcastic letter Edward II had sent to his brother Louis Beaumont, bishop of Durham, on 10 February 1323: "The king remembers that Richard, the bishop's predecessor, was frequently reproached by Henry de Bello Monte [Beaumont], the present bishop's brother, and other friends and relations for causing by his negligence the wasting of the bishopric by Scotch rebels...but the king knows actually that greater damage is done in the bishopric by the bishop's default, negligence and laziness than in the time of his predecessor, neither the bishop, nor his friends or relations giving counsel or aid according to their promises."  Given that Edward II himself had few equals when it came to negligence and laziness, especially when it came to defending his subjects against Scottish raids, there is much of the pot calling the kettle black about this letter.
Henry did not remain long in custody, however, and Edward trusted him enough in 1324 to send him as an envoy to France. He was still sufficiently in favour with the king in September 1325 to travel abroad with Edward's son, and witnessed the young duke of Aquitaine performing homage to Charles IV on 24 September. Unlike other opponents of the king, however, Henry did not remain in France with Queen Isabella, but returned to England, a bad mistake: the Sempringham annalist says he was imprisoned at Kenilworth Castle in February 1326 "because he would not swear to the king and to Sir Hugh Despenser the son, to be of their part to live and die," a story confirmed by the Croniques de London and the judgement on Hugh Despenser the Younger.  Henry was certainly in prison at Warwick Castle in early August 1326. 
Henry must have been released soon after Isabella and Roger Mortimer's invasion force arrived in the autumn of 1326, and joined the queen at Gloucester in mid-October. The much later chronicler Jean Froissart, who is hopelessly unreliable for Edward II's reign, has a hilariously inaccurate story of Edward and Hugh Despenser the Younger being stuck in their boat within a mile of Bristol Castle for a full eleven days, unable to move because their sins weighed so heavily on them, until finally Henry Beaumont sallied forth in a barge to capture them. As Froissart also says that Edward and his favourite witnessed the execution of Despenser the Elder and the earl of Arundel at Bristol, which they certainly didn't (Arundel was executed in Hereford anyway), that Despenser was ninety at the time of his death - he was actually sixty-five - and that Henry was the "son of the viscount of Beaumont in England," obviously his testimony is not worth very much. 
Henry was much in favour with the new regime after late 1326, and can hardly be blamed for his abandonment of Edward II, who had imprisoned him. But Isabella and Roger Mortimer were unable to hold his loyalty for more than two years. Henry was infuriated by their treaty with Robert Bruce in 1328, which acknowledged Bruce as king of Scots and meant that he would never be able to claim his earldom or his lands. He joined the earl of Lancaster's unsuccessful rebellion against the queen and her favourite in late 1328, and was one of the four men specifically excluded from a pardon in early 1329. (Another was William Trussell, who had read out the charges against Despenser the Younger.) With a death sentence hanging over his head, Henry fled abroad.
Henry became embroiled in the earl of Kent's 1330 plot to restore Edward II to the throne, and met Kent in the duke of Brabant's chamber in Paris. Even after Kent's execution in March 1330, Henry continued plotting against Isabella and Mortimer, and travelled to Brabant, a safe haven - the duke was Edward II's nephew, and Edward's sister Margaret, the dowager duchess, was still alive - and with his allies planned an invasion of England. Isabella and Mortimer were forced to raise soldiers up and down the country to repel the invasion, though in the end it never took place, perhaps because the exiles couldn't afford to pay for it, or because they couldn't stomach the notion of fighting against Edward III.  They had no quarrel with the young king, only with the pair ruling England in his name.
After Edward III overthrew his mother and her favourite in October 1330, he recalled Henry and the other exiles to England, and restored their lands. Henry was subsequently to play a vital role in the king's Scottish wars of the 1330s, but as I know next to nothing about Edward III's Scottish wars, I won't embarrass myself by trying to write about them.
Henry Beaumont died shortly before 10 March 1340, aged about sixty, leaving his widow Alice, who lived until 1349. They had three children: John, Lord Beaumont, killed in a jousting tournament in Northampton in 1342, who married Henry of Lancaster's daughter Eleanor (she married secondly the earl of Arundel); Isabella, who married Henry of Lancaster's son Henry, duke of Lancaster; and Katherine, who married David de Strathbogie, earl of Atholl. All his children have numerous modern-day descendants, and through his daughter Isabella, Henry Beaumont was the great-grandfather of Henry IV.
1) Cal Close Rolls 1318-23, p. 717.
2) Cal Close Rolls 1318-23, p. 697.
3) Le Livere de Reis de Britanie e le Livere de Reis de Engletere, ed. John Glover, pp. 354-5; Croniques de London depuis l’an 44 Hen III jusqu'à l'an 17 Edw III, ed. J. G. Aungier, p. 49; G. A. Holmes, ‘The Judgement on the Younger Despenser, 1326’, English Historical Review, 70 (1955) p. 266.
4) Cal Close Rolls 1323-27, p. 593; Cal Fine Rolls 1319-27, p. 418.
5) Froissart: Chronicles, ed. Geoffrey Brereton, pp. 41-3.
6) Cal Pat Rolls 1327-30, pp. 544, 563, 570-572; Cal Close Rolls 1330-33, pp. 51, 147, 151.