Second Baron Montacute. Participant in Scottish and Welsh Norman wars, peace negotiator.

Served against Scots in 1301 and 1304; Imprisoned in the Tower for treason, 1304; knighted along with Prince Edward, 1306; with Edward in Scotland in 1306-1307, also in 1311; in command of the fleet, 1313; with king and queen to coronation of Louis X of France, 1313; Scotland, 1314; commander in campaign (and negotiated peace with) Llywelyn ab Rhys in Wales, 1316; sent to Scotland to negotiate peace, 1317.

Seneschal of Aquitane and Gascony and governor of island of Oleron, 1318 (these were then English possessions in France); commended by the king to the king of France on his appointment to France; died in France.

A senseschal is a "stand-in" for the real lord, that is, someone who administrates the holding of an absentee lord (and who has the powers of justice of that lord, but must also fulfill the feudal obligations of that lord). Perhaps this appointment was an effort to placate the drift toward the Hundred Years war   [1]   [2] ? The English king, in theory, was a vassal of the French king (but the English king had a claim on the inheritance of the French throne...).

The campaign against Llwelyn is described in Vita Edwardi Secundi by the Monk of Malmesbury:

"Meanwhile Llewelyn violently attacked the district under Payn's protection, killing, burning, and plundering. He had attracted to himself some 10,000 Welshmen, who had carried off all Payn's goods to the mountains...

The earl went off to his lands to pursue Llewelyn, and both lords Roger Mortimer helped him. William de Montague commander of the royal cavalry came from one side, and John Giffard and his company from the other. Henry of Lancaster and the other barons and knights who had lands in the neighborhood, brought help, so that the Welsh hemmed in on all sides had no place of refuge at all. Llewelyn therefore, seeing that he had made a bad mistake, and that he was powerless to resist, for the Welsh had twice or thrice met the English on plundering raids but had always had the worst of the encounters, offered to submit to the earl on the condition that he should have safety of life and limb, lands and movables, and in satisfaction for his crime he offered a great sum of money, but the earl would not receive him unless he would surrender unconditionally. At length when our army had drawn near and had already recognized the Welsh guards, Llewelyn began to address his men thus: "It is not safe," he said, "to engage the English; I provoked this business; I will hand myself over for the whole people: it is better that one man should die than that the whole race should be exiled or perish by the sword." ( Vita Edwardi Secundi, Denhom-Young trans.).

Although the translator uses "Montague" throughout, the original Latin reads Willelmus de Monte Acuto and Willelmus de Mountagu. Denhom-Young, the translator, also notes:

" Sir William Montague, of Somerset and Devon (Dugdale, Baronage, i, `640', `633' recte 644, 645) had left an account of his doings against Llewelyn Bren in the Black Mountains in Glamorgan ( Cal. Chanc. Warr. I 437-439; March 1316). He was steward of the household in 1316-18 at 200 marks a year. His son, who held the famous tournament in Cheapside in 1331 ( Ann. Paul. 354) was made Earl of Salisbury in 1337."(Denhom-Young).

Sources:
The Dictionary of National Biography.
Vita Edwardi Secundi, N. Denhom-Young, trans.

http://www.montaguemillennium.com/familyresearch/h_1319_william.htm