Arnulf's father was a leading magnate in Normandy and England, and played an active part in the Anglo-Norman invasion of Wales in the late eleventh century.

Not long after reaching this apex of his career, Arnulf assisted his eldest surviving brother, Robert de Bellême, in a rebellion against Henry I, King of England.

It was also about this time that Arnulf married a daughter of Muirchertach Ua Briain, King of Munster, in what appears to have been an effort to gain military support against the English Crown.

Following the ultimate collapse of the rebellion, however, the Montgomerys were outlawed and banished from the realm, and Arnulf appears to have spent much of the next twenty-odd years in a peripatetic life in Ireland and Normandy.

When William returned to Normandy as king in 1067, Roger accompanied him back to England, and was granted extensive lands, including the Sussex rapes of Arundel and Chichester, followed by the county of Shropshire.

[14] Arnulf makes his first appearance in the historical record at about this time when he and his elder brother, Roger de Poitou, witnessed William's confirmation of their father's grant to the Norman abbey of Troarn in 1082/1083.

[21] According to the "E" version of the Anglo-Saxon Chronicle—the only strictly contemporary source of the four—Robert Curthose's followers captured Odo, Bishop of Bayeux and thereby gained control of the castle of Rochester.

[56] In a Latin grant to the abbey of Saint-Martin de Sées,[53] founded by his father,[57] Arnulf bestowed a donation on behalf of his ancestors, lord, friends, and "very dear brother Hugh" ("carissimi fratris sui Hugonis").

[68][note 8] In fact, the ravaging of the lands of St Davids in 1097 by Arnulf's steward at Pembroke, Gerald, is recorded by the thirteenth- and fourteenth-century Brenhinedd y Saesson, Brut y Tywysogyon, the "B" and "C" versions of the eleventh- to thirteenth-century Annales Cambriæ.

[72] Arnulf appears to have learned of his brother's fate about a month later in Normandy, since he travelled to Sées, and founded a priory of the abbey's monks at Pembroke in dedication to the memory of Hugh de Montgomery and his father.

[81] His brother, Roger de Poitou, was one of the most powerful magnates in northwest England,[82] holding lands in Lancashire, Nottinghamshire, Essex, Yorkshire, and Suffolk.

[86][note 11] In August 1100, whilst Robert Curthose was absent en route from the Holy Land, the reigning William Rufus was killed, and the vacant English throne was seized by their younger brother, Henry, Count of the Cotentin.

[102] Specifically, Brut y Tywysogyon reveals that Arnulf sent Gerald of Windsor to Ireland in order to arrange military assistance from Muirchertach Ua Briain, King of Munster.

[103] The alliance was formalised by a remarkable marriage between Arnulf and one of Muirchertach's daughters, the record of which is preserved by Historia ecclesiastica,[104] Brenhinedd y Saesson,[105] Brut y Tywysogyon,[106] and alluded to by the eleventh- to fourteenth-century Annals of Inisfallen.

[127][note 15] Despite the fact that Brut y Tywysogyon reports that Muirchertach lent the two brothers military support,[131] and the distinct possibility that Roger de Poitou aided them as well,[132] the Bellême-Montgomery insurrection ended in utter failure.

[136] According to the twelfth-century Historia regum Anglorum and Chronicon ex chronicis, Robert de Bellême and Arnulf, supported by Welsh allies, ravaged a part of the county, before carrying off livestock and men to Wales.

This source reveals that, following Robert de Bellême's flight from the king's summons, Henry appears to have raised a feudal host consisting of his tenants-in-chief (who owed him knight-service) and the old English fyrd (a levy of one armed man from about every five hides or six carucates).

Upon being rebuffed by the latter, William Pantulf is recorded to have gone over to Henry's side, and was apparently instrumental in convincing Robert de Bellême's Welsh allies desert him and support the king instead.

[142][note 17] Brut y Tywysogyon specifically states that Iorwerth ap Bleddyn, a leading Welshman, was bought off by the king and began to harry his former ally's lands.

[170][note 22] If there is any truth to the claim of Arnulf being driven from Ireland, it is possible that Historia ecclesiastica may evince an attempt by Muirchertach to mend fractured relations with Henry.

[181] Evidence that he visited England, at least on one occasion, may be preserved by Vita Anselmi which states that Arnulf made a returning voyage from Normandy ("de Normannia Angliam rediens").

[182][note 23] Between 1110 and 1112, Robert de Bellême involved himself in uprisings in southern Normandy, encouraged by Henry's opponent, the recently inaugurated Foulques, Count of Anjou and Maine.

[10] That year the townsfolk of Alençon rebelled against Henry and their lord Stephen, Count of Mortain,[189] whilst the latter were campaigning against a continental coalition attempting to replace the king with Robert Curthose's illegitimate son, William Clito.

[190][note 24] The region of Alençon was a former power centre of the Bellême family, and according to Historia ecclesiastica, the townsfolk requested that Arnulf intervene with Foulques on their behalf against Stephen's injustices and oppression.

This insurrection seems to have contributed to Henry's restoration of much of the former Montgomery-Bellême lands in Normandy to Robert de Bellême's son, William III, Count of Ponthieu, in June 1119.

[196] The next certain record concerning Arnulf occurs in 1122, when his name is listed in a mortuary roll, circulated after the death of churchman Vitalis of Savigny, in which the nuns of the abbey of Alménêches commemorated him, his parents, and his younger brother Philip.

[207][note 26] Historia ecclesiastica describes Hugh de Montgomery as the only "mild and loveable" (mansuetus et amabilis) of Mabel's sons,[209] whilst Welsh sources present him in a much more negative light.

This source—a contemporary composition bewailing the cultural upheaval and oppression inflicted upon the Welsh after the Anglo-Norman conquests of 1093—may refer to subjugation suffered under Arnulf and his father.

Although Norman families tended to practice primogeniture, the conquest of England and the opening up of Britain contributed a new area of exploitation for landless younger sons of the aristocracy.

The younger sons of Roger de Montgomery and Mabel are an exception,[95] and Arnulf's career illustrates the various opportunities available to contemporaries of his rank—men who could not rely upon inheritance alone, and who had to acquire territories of their own.