Henry was not satisfied with his lot and it has been conjectured that it was he who masterminded the death of William 11, making it look like he was killed as the result of a hunting accident. If this is true then it’s timing was critical. In 1096 Robert of Normandy had joined the Crusade to the Holy Land and had pledged the duchy to William. By the summer of I 100 news reached England that Robert was returning, along with a new bride. Immediately upon the death of William a hastily convened council elected Henry as his successor. This despite the support that many barons had for Robert, who was on a crest of popularity following his victories in the Holy Land, even though his past record showed him as a weak ruler of Normandy. By the first week of September, when Robert had returned to Normandy, Henry had been elected and crowned. One of his first acts was to recall Anselm from his exile to the archbishopric of Canterbury, and with Anselm's support Henry's position was inviolable. He further cemented it by a political marriage to Edith, the daughter of Malcolm III of Scotland and the niece of Edgar the Atheling, thus establishing alliances with the elder Saxon aristocracy and with the Scots.

Robert raised a considerable army and invaded England in June 1101, cleverly misleading Henry whose army waited at Arundel while Robert landed at Portsmouth. It is possible that had Robert pressed home his advantage he could have defeated Henry's army. He might easily have captured Winchester, where the Treasury was held, but his army passed by that town. It stopped short of invading London, though this too was within his grasp. Instead the two armies met at Alton where Robert asked for negotiations. Clearly Robert lacked the opportunism that marked the success of his father and younger brother. Even worse, he was prepared to trust Henry. The result was that Henry agreed to pay Robert 3,000 marks annually and recognize him as the legal claimant to the throne in exchange for Henry remaining king while he lived. In the eyes of Henry and the barons possession was nine-tenths of the law, and Robert was the loser. A few years later, in 1106, Henry took control of the matter, invading Normandy and capturing his brother at Tinchebrai. Robert was brought to England and imprisoned for the rest of his life, which lasted another twenty-eight years: he was certainly over 80 when he died in II 34. Had he succeeded to the English throne in 1087, on the death of his father, he would have ruled for 47 years, one of the longest reigns of an adult monarch. However his weak nature suggests that he would have been overthrown by someone long before his death, and in all likelihood that would still have been his scheming brother Henry.

Whilst Henry was endeavouring to regain Normandy he had troubles at home with the church. Although he had recalled Anselm as archbishop of Canterbury, the relations between the two rapidly deteriorated. Anselm had fallen out with William Rufus because the latter had refused to acknowledge the authority of Rome and Anselm's rights in the reorganization of the church. Anselm reminded Henry of the papal authority in appointing clergy, since the pope had decreed as far back as 1059 that lay investiture was unlawful in the eyes of the Church. Henry would have none of this, and with other matters more pressing refused to consider it. By 1103 Anselm found his position untenable and he again went into exile. The pope threatened to excommunicate Henry and, fearful of how this would undermine his authority as king, Henry recalled Anselm and sought to negotiate a compromise. The result was that in 1106 Henry accepted clerical authority in investiture on the understanding that the clergy still recognized secular authority over the lands owned by the church. In this way Henry kept his revenues (which Anselm had maintained belonged to the church and thus to Rome) and it meant he could still agree who had possession of the property. (It was this loophole that allowed HENRY II to challenge Thomas Becket sixty years later.) Nevertheless when Anselm died in 1109 Henry succeeded in keeping the see of Canterbury vacant for five years.

Once Henry had secured the Dukedom of Normandy he had his hands full in keeping it. Since England was now relatively safe, he found he had to spend more time in Normandy. His queen Matilda officially served as regent during these absences, but increasingly the administration came under the capable control of Roger, bishop of Salisbury. Since Henry drew heavily upon the English revenues to finance his army in Normandy as well as his extensive building projects across England, Roger developed a system for controlling the exchequer. In effect he established the basis for what would evolve into the civil service.

Although Henry would enter into battle if necessary, he sought to pave the way by treaty or diplomacy first, and in this he was admirably skilled. One such act was the marriage in January 1114 of his eldest daughter Adelaide (who adopted the name Matilda upon her marriage) to Heinrich V, Emperor of Germany, and she was crowned Empress on the same day. She was eleven years old; the Emperor was 32. Henry held Normandy against all opposition. His ultimate victory was the defeat of Louis VI of France in 1119. When peace was agreed with the pope's blessing, Henry was accepted unchallenged as duke of Normandy. Henry cemented this advance by marrying his eldest son William to Alice (who also changed her name to Matilda), the daughter of Fulk V, count of Anjou and Maine. William was only fifteen, Alice less than twelve. In 1120, as Henry's eldest son William came of age, he was made duke of Normandy, and stood in succession to the throne of England, even though Henry's eldest brother Robert and his son William were both still alive.

In the summer of 1120 Henry could be proud of his achievements. Through his own marriage and those of his children he had alliances with the strongest neighbouring royal families of Europe; others he had dominated by conquest or treaty. He had reached a satisfactory arrangement with the papacy and all looked well for the future. And then everything fell about him. In November 1120 his two eldest legitimate sons William and Richard drowned when the White Ship foundered off Barfleur while sailing from Normandy to England. He was left without a male heir, although his eldest illegitimate son, Robert Fitzroy, earl of Gloucester, now turned an eye to the throne. Henry's first wife, Matilda had died in May 1118, an event over which Henry did not seem especially concerned. He arranged a quick marriage of convenience to Adeliza, daughter of Geoffrey VII, count of Louvain. That marriage was childless, although Henry had several more illegitimate children, and Adeliza bore seven children to her second husband, William d'Albini, earl of Arundel, after Henry's death.

In 1125 Henry's daughter, Matilda, became a widow when the Emperor Heinrich died. She was twenty-three but had no children. In II 26, fearing he would have no further children, Henry made the barons swear an oath of fealty to Matilda as the heir-presumptive to the throne. The barons agreed, though the idea of being ruled by a queen was detested by them. The position was further aggravated when, in May 1127, Henry arranged a second marriage for Matilda, this time with Geoffrey of Anjou, who was then only fourteen. The Normans had little affection for the Angevins and did not like to consider that Geoffrey might become their King. They began to turn their allegiance to Henry's nephew, William, the son of Duke Robert, who was known as William Clito. At this time, April 1127, he was supported by the French king, who had just made him Count of Flanders. In January 1128 he married Giovanna, the daughter of the count of Burgundy. William was gradually rising in power and his right to the English throne was becoming increasingly recognized by the Norman aristocracy. Unfortunately William was wounded in a skirmish near St Omer in July 1128 and died five days later.

The Barons now realised that there was little alternative but to Matilda becoming their queen, but they increasingly showed their opposition. Geoffrey, who became count of Anjou in 1129, recognized this and though he never seems to have considered himself having any claim on the throne of England, he did consider the Duchy of Normandy and asked Henry if he would give him custody of the castles along the French coast. Henry refused, with the result that the relationship between Henry and Geoffrey deteriorated rapidly. It had not been helped by Matilda deciding she could not abide Geoffrey and deserting him to return to England. Henry, still with an eye on Matilda producing a grandson, sought to reconcile the two with some degree of success. Nevertheless, by 1135 Henry and Geoffrey were openly at war. Henry sailed to Normandy but soon after his arrival he became ill, apparently after eating some lampreys. The result was ptomaine poisoning and six days later Henry died. Despite having declared Matilda his heir, neither she nor the barons took up that position, and the throne was claimed by Henry's nephew, Stephen.

Henry was a highly capable king. Although he was frequently involved in warfare, either direct or diplomatic, most of this was over his lands in Normandy. For all of his long reign, after the first year, he maintained peace throughout England, though it was a peace at the cost of exacting taxes needed to maintain his army. This led to him establishing the crown exchequer, the basis of the future Treasury. With his many campaigns and international affairs, Henry had little time for any-thing other than pleasures of the flesh, although he did establish a royal menagerie at his manor at Woodstock, near Oxford, which is regarded as the first English zoo. It is ironic that, despite having fathered at least twenty-nine children, he was only able to leave one legitimate heir to the throne, and she was not seriously considered by Henry's barons until it became convenient to do so. For all of Henry's schemes and plans during his thirty-five year reign, the longest of any king of England since Athelred II, it all came to nought. Perhaps he schemed too much, for his efforts in his final years to find a successor meant that he found one too many, and within four years of his death, England was plunged into civil war.