Robert, who invaded in 1101, disputed Henry's control of England; this military campaign ended in a negotiated settlement that confirmed Henry as king. The peace was short-lived, and Henry invaded the Duchy of Normandy in 1105 and 1106, finally defeating Robert at the Battle of Tinchebray. Henry kept Robert imprisoned for the rest of his life. Henry's control of Normandy was challenged by Louis VI of France, Baldwin VII of Flanders and Fulk V of Anjou, who promoted the rival claims of Robert's son, William Clito, and supported a major rebellion in the Duchy between 1116 and 1119. Following Henry's victory at the Battle of Brémule, a favourable peace settlement was agreed with Louis in 1120.
Considered by contemporaries to be a harsh but effective ruler, Henry skilfully manipulated the barons in England and Normandy. In England, he drew on the existing Anglo-Saxon system of justice, local government and taxation, but also strengthened it with additional institutions, including the royal exchequer and itinerant justices. Normandy was also governed through a growing system of justices and an exchequer. Many of the officials who ran Henry's system were "new men" of obscure backgrounds rather than from families of high status, who rose through the ranks as administrators. Henry encouraged ecclesiastical reform, but became embroiled in a serious dispute in 1101 with Archbishop Anselm of Canterbury, which was resolved through a compromise solution in 1105. He supported the Cluniac order and played a major role in the selection of the senior clergy in England and Normandy.
Henry's only legitimate son and heir, William Adelin, drowned in the White Ship disaster of 1120, throwing the royal succession into doubt. Henry took a second wife, Adeliza, in the hope of having another son, but their marriage was childless. In response to this, Henry declared his daughter, Matilda, his heir and married her to Geoffrey of Anjou. The relationship between Henry and the couple became strained, and fighting broke out along the border with Anjou. Henry died on 1 December 1135 after a week of illness. Despite his plans for Matilda, the King was succeeded by his nephew, Stephen of Blois, resulting in a period of civil war known as the Anarchy.
Henry was probably born in England in 1068, in either the summer or the last weeks of the year, possibly in the town of Selby in Yorkshire. His father was William the Conqueror, the Duke of Normandy, who had invaded England in 1066 to become the King of England, establishing lands stretching into Wales. The invasion had created an Anglo-Norman elite, many with estates spread across both sides of the English Channel. These Anglo-Norman barons typically had close links to the kingdom of France, which was then a loose collection of counties and smaller polities, under only the minimal control of the king. Henry's mother, Matilda of Flanders, was the granddaughter of Robert II of France, and she probably named Henry after her uncle, King Henry I of France.
Henry was the youngest of William and Matilda's four sons. Physically he resembled his older brothers Robert Curthose, Richard and William Rufus, being, as historian David Carpenter describes, "short, stocky and barrel-chested," with black hair. As a result of their age differences and Richard's early death, Henry would have probably seen relatively little of his older brothers. He probably knew his sister, Adela, well, as the two were close in age. There is little documentary evidence for his early years; historians Warren Hollister and Kathleen Thompson suggest he was brought up predominantly in England, while Judith Green argues he was initially brought up in the Duchy. He was probably educated by the Church, possibly by Bishop Osmund, the King's chancellor, at Salisbury Cathedral; it is uncertain if this indicated an intent by his parents for Henry to become a member of the clergy. It is also uncertain how far Henry's education extended, but he was probably able to read Latin and had some background in the liberal arts. He was given military training by an instructor called Robert Achard, and Henry was knighted by his father on 24 May 1086.
In 1087, William was fatally injured during a campaign in the Vexin. Henry joined his dying father near Rouen in September, where the King partitioned his possessions among his sons. The rules of succession in western Europe at the time were uncertain; in some parts of France, primogeniture, in which the eldest son would inherit a title, was growing in popularity. In other parts of Europe, including Normandy and England, the tradition was for lands to be divided up, with the eldest son taking patrimonial lands – usually considered to be the most valuable – and younger sons given smaller, or more recently acquired, partitions or estates.
In dividing his lands, William appears to have followed the Norman tradition, distinguishing between Normandy, which he had inherited, and England, which he had acquired through war. William's second son, Richard, had died in a hunting accident, leaving Henry and his two brothers to inherit William's estate. Robert, the eldest, despite being in armed rebellion against his father at the time of his death, received Normandy. England was given to William Rufus, who was in favour with the dying king. Henry was given a large sum of money, usually reported as £5,000, with the expectation that he would also be given his mother's modest set of lands in Buckinghamshire and Gloucestershire. William's funeral at Caen was marred by angry complaints from a local man, and Henry may have been responsible for resolving the dispute by buying off the protester with silver.