Salisbury was an important general for Henry V. Like Bedford he remained in France. He is fatally wounded early in the play, but his valor shines through.
The following information is quoted from the Oxford Dictionary of National Biography, written by Anne Curry.
Montagu, Thomas [Thomas de Montacute], fourth earl of Salisbury (1388–1428), soldier, was the elder son of John Montagu, third earl of Salisbury (d. 1400), and his wife, Maud Montagu (d. 1424), daughter of Adam Fraunceys, mayor of London, and widow of John Aubrey, and of Sir Alan Buxhull (d. 1381). Earl John died in the aftermath of the abortive plot to murder Henry IV, planned for 4 January 1400. Despite his father's subsequent attainder for treason in 1401, Thomas Montagu was granted an annuity out of the comital lands for his maintenance, and was already styling himself earl. On 14 June 1409 he proved his age, performed fealty, and was restored to all that his father had held in fee tail. In October next he was summoned to parliament as the earl of Salisbury, and in 1414 he petitioned for full restoration, to which the king all but acquiesced in 1421 after the Commons had added their support.
The reason for this return to royal favour was Salisbury's service in France in which he was engaged almost continuously under Henry V and Henry VI, thus playing little role in domestic high politics. In 1412 he served on the expedition sent under Thomas, duke of Clarence. Created a knight of the Garter in April 1414, he was involved in negotiations with the French and discussions in the royal council about Gascony before serving on the Agincourt campaign. In 1416 he was assigned with Humphrey, duke of Gloucester, to greet Emperor Sigismund at Dover, and also served on the naval expedition under John, duke of Bedford. In the following year he crossed with the king for the conquest of Normandy, where he took part in Henry's sieges of Caen and Falaise before accompanying Clarence on an expedition along the Risle. At the siege of Rouen he was stationed at the Mont-St Catherine. He was later responsible for the capture of Fécamp, Montivilliers, Gournay, Eu, and Honfleur (of which he was captain, 1419–20). His military prominence is confirmed by his appointment on 26 April 1419 as the king's lieutenant-general in Normandy and in the marches south of the Seine. On 13 November 1420 his charge was redefined as the whole duchy in addition to Maine. The level of royal trust is also revealed by his involvement in diplomatic activities in the autumn of 1418 and spring of 1419. Having accompanied Henry V on his entry to Paris on 1 December 1420, he was appointed on 25 December to the governorship of Alençon, Essay, Exmes, Bonsmoulins, and Verneuil, places he continued to control until Michaelmas 1423. He also benefited from royal largess under Henry and his son: he was created count of Perche on 26 April 1419, and enjoyed a series of major land grants in France between 1418 and 1427.
Salisbury accompanied Clarence on his campaign into Maine and Anjou in the spring of 1421. According to Jean de Waurin (d. 1473/4), the duke advanced against the French and Scots at Baugé without waiting for the rear guard under Salisbury. All the earl could do was to drive off the victorious French in order to retrieve Clarence's body, and to conduct his own brief raid into Anjou. He continued in service in France after the death of Henry V, assisting in the recovery of Meulan and Orsay, and holding the captaincy of Falaise from 25 December 1422 to his death. In June 1423 Bedford appointed him governor of Champagne, Brie, Auxerrois, Nivernois, and the Mâconnais, and in this capacity he laid siege to Montaguillon, near Provins. When the French army of relief advanced against Cravant, Salisbury was sent with reinforcements to assist the Burgundians, and together they defeated the French on 30 July 1423, with the earl in command of the left flank. He continued to campaign successfully in Champagne over the next year. In preparation for the journée of Verneuil, John, duke of Bedford, ordered Salisbury to take up an initial position four leagues from the enemy. Waurin, who was an eyewitness, claimed that without the earl's personal example of valour, which spurred on those who fought under his banner, the battle might have gone quite differently. In September and October 1424 he attempted with William de la Pole, earl of Suffolk (d. 1450), and reinforcements from England to recover Nogent le Rotrou, Senonches, Rambouillet, and Rochefort. He afterwards took over the captaincies of Nogent and Montigny. By the summer of 1425 the situation in Champagne was secure enough to allow him to take up command of the advance into Maine and Anjou. He conducted a highly successful campaign whereby all of the key places, including Le Mans, were captured. At Bedford's departure for England in September 1425 conduct of the war was entrusted to Richard Beauchamp, earl of Warwick, and the earls of Suffolk and Salisbury, with the latter being given charge of much of Normandy as well as Anjou, Maine, the Vendômois, the Chartrain, and Beauce. Warwick was assigned to the ‘marches et pays de France’, thus replacing Salisbury in the theatre of the Vermandois, Champagne, Brie, and the Gâtinais. In February 1426, however, Salisbury resigned his command to go on pilgrimage to Jerusalem in accordance with a vow made during the course of a battle. By July he had changed his mind; he obtained a dispensation from the pope which released him from his vow, and returned to the field, resuming his activities in Champagne while Warwick maintained the advance into Maine. From October 1426 Salisbury also held the captaincy of Meulan. When Bedford returned to France in April 1427, Warwick was appointed lieutenant of Normandy, Anjou, Maine, and the marches of Brittany while Salisbury returned to England, perhaps in search of reinforcements. His return had been anticipated by his appointment as a privy councillor in November 1426 and he attended the council regularly from July 1427 to 1 June 1428, as well as the parliament of 1427–8, where he took the opportunity to present a joint petition with Gloucester concerning wages still owing from the Agincourt campaign. His stay was dominated by the planning and raising of a major expeditionary force with which he sailed in July 1428.
Bedford had hoped to use Salisbury's reinforcements along with troops raised within Normandy and France for a final thrust into his own duchy of Anjou, but the campaign which ensued was directed instead towards Orléans, perhaps with its ultimate goal an assault on Bourges. Why the change in plan occurred and whether Salisbury was instrumental in it have been the subject of much debate. The attack on Orléans was most certainly against the interests and the will of the duke of Burgundy, Philip the Good, who, according to Fenin, had made amorous advances towards the countess of Salisbury during the wedding celebrations of the seigneur de Toulongeon in Paris in November 1424, offending the earl and leading him to support Gloucester's anti-Burgundian activities in the Low Countries. Further clashes with Philip had arisen over the administration of Champagne. Salisbury was also in dispute with Bedford over rights in the duchy of Alençon, but it is perhaps going too far to see the earl deliberately acting against Bedford in the war policy of 1428. The earl met Bedford at Paris before conducting a swift and successful campaign southwards, skilfully establishing a forward base at Janville and isolating Orléans by capturing the Loire towns of Jargeau, Beaugency, and Meung. By 12 October he had drawn his army up to besiege Orléans and within eleven days had forced the French to abandon the fortification of Les Tourelles which controlled the bridge. While observing the city from Les Tourelles on 27 October he was seriously wounded in the face by a cannon shot from the city. Salisbury's death at Meung on 3 November 1428 deprived the English of their most talented and experienced commander. His body was taken back to England; after a mass at St Paul's on 29 November he was buried at Bisham Abbey in accordance with his will.
Salisbury married twice. His first wife, whom he married on or before 23 May 1399, was Eleanor, daughter of Thomas Holland, earl of Kent (d. 1400), and Alice, daughter of Richard (III) Fitzalan, earl of Arundel. Neither Eleanor's date of death nor the date of Salisbury's marriage to his second wife, Alice Chaucer (c.1404–1475), daughter of Thomas Chaucer (d. 1434) and Maud Burghersh (d. 1437), is known. Eleanor was definitely dead by 10 June 1424; thus it must have been his second wife, Alice, who aroused the passions of the duke of Burgundy in November 1424. It has been suggested that Salisbury may already have been married, or at least contracted, to Alice by May 1421 when her father was speaker of the parliament in which the Commons petitioned with some success for the full restoration of the earl's lands. Alice had been married as a child to Sir John Philip who died at Harfleur in 1415 and was sole heir to her father's fortune. After Salisbury's death she married, in November 1430, William de la Pole, earl of Suffolk (d. 1450). With his first wife, Salisbury had one daughter, Alice (c.1406–1462), who married in or before February 1421 Richard Neville (d. 1460), younger son of Ralph Neville, earl of Westmorland, and Joan Beaufort. Neville seems to have been styled earl of Salisbury from the time of his father-in-law's death; the privy council confirmed his right to the earldom in May 1429. There was also an illegitimate son, Sir John Montagu (Bastard Salisbury), who served in France; he was captain of Gournay and Gerberoy (1430–31), of Argentan (1431–4), lieutenant at St Lô (1434–5), and captain of Fresnay (1446–8).