MONTACUTE or MONTAGU, WILLIAM de, third Baron Montacute and first Earl of Salisbury (1301–1344), born in 1301, was eldest son of William de Montacute, second baron Montacute (d. 1319) [q. v.], and succeeded his father as third baron on 6 Nov. 1319, being granted wardship of his own lands, though yet a minor. In 1322 he came of age, and received livery of his lands, together with the grant of Lundy Isle. In 1325 he was knighted, and received letters of protection on his departure for France (Rymer, ii. i. 606). In 1327 he went with Edward III to repel the Scottish invasion, when the latter nearly missed capture. In 1329 he accompanied the king abroad and was sent in June to treat for a marriage between the eldest son of the king of France and Edward's sister Alianore (ib. ii. ii. 764, 766). In September he was despatched with Bartholomew de Burghersh (d. 1355) [q. v.] on an embassy to the pope at Avignon, returning before the end of the year, when, in his capacity as executor of Blanche, queen of Navarre, he lent the king two thousand marks that had belonged to her, and were deposited at Whitefriars.
Next year the young king took him into his confidence about his plans for the arrest of Mortimer. During the parliament held at Nottingham in October 1330, Montacute, with a band of retainers, including Sir John de Molines [q. v.], penetrated by a secret passage into the castle, where they found Mortimer in the queen-mother's apartments (Murimuth, p. 61). After a struggle, in which two of Mortimer's attendants were killed, his arrest was effected, and he was sent to London for trial [see Mortimer, Roger IV de, first Earl of March; and Barnes, Edward III, pp. 47-8]. Edward obtained from parliament indemnity on Montacute's behalf for all consequences of the death of Mortimer's attendants, and rewarded him with various grants of land forfeited by Mortimer in Hampshire, Berkshire, Buckinghamshire, Kent, and Wales, including Sherborne, Corfe Castle, and Purbeck Chase in Dorset, and the lordship of Denbigh (Rolls of Parl. ii. 60 b; Galfridi le Baker, Chron. ed. Maunde Thompson, pp. 46, 226-8; Walsingham, Ypodigma Neustriæ, p. 270; Murimuth, pp. 62, 285; Dugdale; Stow, Annals, p. 229; Stubbs, ii. 390; Longman, Edward III, i. 35). On 4 April 1331 Montacute accompanied Edward III when, disguised as a merchant and attended by a handful of men-at-arms, the king paid a secret visit to France; he was present when Edward repeated his homage to the French king at Amiens on 13 April, and returned with him to Dover on 20 April (Froissart, ed. Lettenhove, ii. 232; Rymer, ii, ii. 818). In September Montacute held a tournament in Cheapside, entertaining his guests in the Bishop of London's palace.
Next year he attended the king in Scotland, and in 1333 was present at the siege of Berwick and the battle of Halidon Hill (Barnes, p. 80); in the same year Edward made over to him all his rights to the Isle of Man. He appears to have accompanied Balliol to Scotland, and in February 1334 was deputed by him to excuse his absence from the parliament held at York. On 30 March Montacute was appointed envoy to France with the Archbishop of Canterbury and two others (Rymer; Barnes, p. 92); but in June was again in Scotland, where in 1335 he was left in command of the army with Arundel. In the same year he was granted the forests of Selkirk and Ettrick and town of Peebles, made governor of the Channel Islands and constable of the Tower. In November he was given power to treat with Andrew Murray, constable of Scotland; on 27 Jan. 1336 he commenced the siege of Dunbar Castle, but after nineteen weeks the blockade was raised by Alexander Ramsay, and Montacute gave it up in despair, making a truce that was strongly disapproved of in England (Walsingham, Ypodigma, p. 275; Hist. Angl. p. 200; Stow, p. 231; Longman, p. 189; Lettenhove, xxiii. 93-7; Barnes, pp. 101 sqq.) In the same year he was appointed admiral of the fleet from the mouth of the Thames westward.
On 16 March 1337, at the parliament held in London, Montacute was created Earl of Salisbury. In the following April he was sent to Philip to declare Edward's claim to the French crown, and thence on an embassy to the emperor Lewis, Rupert, Count Palatine, the Duke of Bavaria, and other princes of Germany and the Netherlands, to organise a league against France (Lettenhove, xxiii. 97; Rymer, ii. ii. 969, 992, 995). In October he was commissioned to treat with Scotland, but in July 1338 commanded a successful raid into Scotland from Carlisle. Later on in the year he sailed with Edward from the Orwell to Flanders, and by a patent, dated Antwerp 20 Sept. 1338 (Rymer), was appointed marshal of England, an office then vacant by the death of Thomas, earl of Norfolk. He remained in Flanders, where he was one of the captains of the English forces, for the next two years, during part of which he was in garrison at Ypres (Lettenhove, passim). In November 1338 he was one of those appointed to treat with Philip of Valois at the desire of the pope; shortly after
