Norman invasion of Wales
The Norman invasion of Wales began shortly after the Norman conquest of England under William the Conqueror, who believed England to be his birthright. It was not William's intention to also invade Wales, but Welsh attacks under King Gruffudd ap Llywelyn, uniter of Wales, against the Normans in England, beginning in the years prior to the main Norman invasion in 1066, eventually forced William's hand. Initially (1067–1081), the invasion of Wales was not undertaken with the fervor and intentionality of the invasion of England. However, a much stronger Norman invasion began in 1081 and by 1094 most of Wales was under the control of William's eldest son, King William II of England. The Welsh greatly disliked the "gratuitously cruel" Normans and by 1101 had regained control of the greater part of their country under the long reign of King Gruffydd ap Cynan, who had been imprisoned by the Normans for twelve years before his escape. Gruffydd had some indirect help from King Magnus III of Norway (Magnus Barefoot) who attacked the Normans briefly off the Isle of Anglesey in Northwest Wales near Ynys Seiriol, killing Hugh of Montgomery, 2nd Earl of Shrewsbury and leaving the Normans depleted and demoralized. Magnus went on to take the Orkney Islands, the Hebrides, and the Isle of Man, islands north of Wales and west or north of Scotland and England, in 1098.
Under William's fourth son, King Henry I of England, the Normans, now well established in England, responded by pushing west into Wales. This time, both the Welsh and the Normans were more interested in making peace than fighting bloody battles, and a relatively stable situation developed, although the Normans fared more poorly in southeast Wales than in the west of the country. The standoff continued from 1135 to 1154 under Stephen, King of the English, nephew of Henry and a maternal grandson of William, who became locked in a power struggle and civil war with Empress Matilda, Henry's daughter and only surviving legitimate child.
In 1157, King Henry II of England, son of Matilda, led his first, relatively successful, expedition into Wales, beginning a stronger display of English/Norman power. In 1163 Henry displayed even greater might, and the two most powerful Welsh princes, Rhys ap Gruffydd and Owain Gwynedd, soon accepted the situation and paid homage to Henry, beginning a period of the domination of Wales by England but with a degree of independence by the Welsh, a situation which has continued, to various extents, to the present day.
Welsh attacks in England 
By the mid-11th century, Wales had been united by a king of Gwynedd, Gruffydd ap Llywelyn. Gruffydd pushed into Saxon England, burning the city of Hereford, overwhelming border patrols, and proving the English entirely inadequate to respond to Welsh invasions. Subsequent to his uniting of Wales and his victories over Mercia and other English kingdoms, Gruffydd was turned upon by his own men, who killed him in 1063 and shipped his head off to Edward the Confessor in exchange for the redivision of Wales into its traditional kingdoms. This left a vacuum of power in Wales in which princes and kings were free to squabble over their land. In addition, they were left without the presence of Gruffudd to ward off English attacks.
Early battles 
It took some time for the Normans to concentrate any level of might against the Welsh, however, as they were more concerned, in the aftermath of Hastings, with England and Normandy. In addition, it was not William’s goal to conquer Wales; he had come to inherit what he believed to be his birthright, the English throne, which entailed taking on the responsibilities of Edward and the Anglo-Saxon kings, including their relationships with Wales and Scotland. However, Wales had begun to force the matter, supporting English rebellions against the Normans.
William's response 
In response to Welsh advances, William established a series of earldoms in the borderlands, specifically at Chester, under Hugh d'Avranches; Shrewsbury, under Roger de Montgomerie; and Hereford, under William FitzOsbern. He instilled a great deal of power into each earldom, allowing them control of the surrounding towns and land, rather than retaining it within the kingship. The inspiration for such an action seems to have been the overextended nature of the Norman troops, thus preventing William from exercising his own power in the area. It very well may have been implicit in the power granted the earldoms that they were to attack Wales, and, indeed, they did, beginning with south-east Wales, where many of the previous rebellions against England had begun. By the time of FitzOsbern's death in 1071, a castle had been established at the mouth of the Wye, and it served as a base from which the Normans continued to expand westward into Wales, establishing a castle at Caerleon by 1086 and extinguishing the Welsh Kingdom of Gwent. However, the attacks in south-east Wales "faltered badly when [the earl of Hereford’s] son [Roger de Breteuil]... forfeited his estates for treason in 1075 and involved some of his vassals on the Welsh frontier in his downfall". Nonetheless, the Normans pressed on.
Relations with Henry I 
These movements continued well into the 12th century, into the reign of Henry I. There was relative peace in the early 12th century, however, with a great deal of English influence over Wales but relatively little conflict. The only real expedition into Wales made by Henry was in 1114, when "he set in motion three separate armies intended to overware the ageing prince of Gwynedd". The prince, Gruffydd ap Cynan, however, saw fit to make peace with the king rather than engage in open warfare or hostility. Throughout the period, Henry exerted a great deal of control over Wales, establishing a series of new castles and placing new Lords into positions of power. Upon Henry’s death, revolts once again broke out in parts of Wales. These revolts caused Norman retreat in many areas, most surprisingly in Deheubarth, where, according to R.R. Davies, "the Normans had made their most striking advances in the previous generation". The period saw a role reversal of sorts, as well, with infighting amongst the Normans, the same sort which had enabled the relative fall of Wales in the previous century.
Invasion and control under Henry II 
By the 1150s, Henry II had set upon fighting back, leading his first expedition into Wales in 1157. Though met with some setbacks, Henry seems to have been primarily successful. He moved into Wales again in 1163, this time showing his true force of political power, forcing homages from the two most powerful princes of Wales, Rhys ap Gruffydd and Owain Gwynedd. Minor changes in power continued into the 13th century, until 1277, with Welsh princes consolidating and retaining their basic political and geographic structure.
See also 
- Norman conquest of England
- Norman invasion of Ireland
- Welsh uprising of 1211
- Conquest of Wales by Edward I
- Davies, R.R.. The Age of Conquest: Wales, 1063-1415, p. 26
- Ibid, p. 24
- Ibid p. 27
- Ibid, p. 28
- John Davies, A History of Wales, 1993, ISBN 0-14-028475-3, pp.100-102
- Ibid, p. 29
- Ibid, p. 41
- Davies, R.R.. The Age of Conquest: Wales, 1063-1415, p. 45
- Ibid, p. 52
- Carr, A.D. (1995). Medieval Wales. St. Martin's Press. ISBN 0-333-54772-1.
- Davies, R.R. (2000). The Age of Conquest: Wales, 1063-1415. St. Martin's Press. ISBN 0-19-820198-2.
- Walker, David (1990). Medieval Wales. Cambridge University Press. ISBN 0-521-31153-5.