Henry first attempted to continue his mother's war against Stephen after she had returned to Normandy in 1148, but Henry was a young squire of fifteen without sufficient resources to maintain such an effort. The next five years would see a significant change in him. When his father died in 1151 he inherited the duchy of Normandy as well as becoming count of Maine and Anjou. Eight months later he married Eleanor of Aquitaine, who was at least ten years his senior, the former wife of Louis VII of France whom Louis had divorced, ostensibly on grounds of consanguinity, but really because she had provided no male heir. This marriage infuriated Louis VII, especially when he had recognize the claim of Henry as duke of Aquitaine. Although Henry paid homage to Louis for his lands in France, he now effectively controlled more territory than the King himself. Louis sent forces against Henry as a show of power but Henry was able to contain them. In fact he felt sufficiently in control to accompany a small force to England in January 1153 in an effort to depose Stephen. In this he was unsuccessful, but Stephen was no longer disposed to fight, and most of the hostilities were between Henry and Stephen's son Eustace. In August 1153 Eustace died and this paved the way for Henry's succession which was sealed under the Treaty of Wallingford that November. By its terms Stephen continued to rule for as long as he lived but Henry was his undisputed successor. When Stephen died in October 1154, Henry succeeded to a considerable territory, subsequently called the Angevin Empire, though not known as that in Henry's day. At its peak it stretched from the Scottish border to the Pyrenees, and would include overlordship of Ireland.

The energy with which Henry set about establishing his authority over his territories was awesome. This was helped by the papal bull issued in 1155 by the new Pope Adrian IV (the only English pope - Nicholas Breakspeare), which decreed that Henry had authority over the whole of Britain, including Scotland, Wales and Ireland. In the space of two years (1155-7) Henry had destroyed many of the castles established by barons during the civil war, and which he referred to as "dens of thieves"; he had negotiated terms with Malcolm IV of Scotland, whereby Cumbria and Northumberland returned to English rule; and he had invaded Wales and brought the Welsh princes to heel. This last enterprise nearly cost him his life, however, when he was ambushed by the heir of Gwynedd, Cynan ab Owain. Henry's strength of character, his papal authority, and the immense resources upon which he could draw made him an impossible man to challenge, and by 1158 he had restored an order to England and its subservient kingdoms which it had not known to such a degree for many lifetimes. Wales would continue to be a thorn in his side for much of his reign, but he never considered it as much of a problem compared to other priorities. Subsequent campaigns of 1167 and 1177 served to remind successive Welsh rulers of his authority.

From 1158 to 1163 Henry was back in France. In July 1158 his brother, Geoffrey, had died. Geoffrey in 1150 had been made count of Nantes, one half of the duchy of Brittany, and on Geoffrey's death Henry sought to gain control. He was foiled by the speed with which the exiled duke, Conan IV, reclaimed his lands. Conan had been confirmed as earl of Richmond by Henry in 1156, and Conan was forced to acknowledge Henry's overlordship in Brittany. In 1166 Henry arranged a marriage between his son Geoffrey and Conan's daughter Constance, and thereafter Conan handed over the administration of Brittany to Henry to direct on behalf of the children. Henry's main thrust during 1159 and 1160 was against Toulouse, which he regarded as part of his wife's territory in Aquitaine. The French king, Louis VII, came to the defence of his brother-in- law, the count of Toulouse, and Henry had to withdraw rather than fight his French overlord. Toulouse and Aquitaine remained in dispute between Henry and Louis for the rest of their reigns.

The most notorious aspect of Henry's reign was his relationship with Thomas Becket. Becket was a personal friend of Henry's. Born in London, the son of a wealthy merchant, he was well educated and had trained as a knight before his father's misfortunes turned him to become a clerk, entering the household of Theobald, the archbishop of Canterbury, in 1142. He became an expert at canonical jurisprudence, and was appointed archdeacon of Canterbury in 1154 and chancellor of England in 1155. He fought alongside Henry in Toulouse and became wealthy. His election as the next archbishop of Canterbury in May 1162 came as a surprise to many, and was not universally accepted amongst other churchmen because of Becket's background and worldliness. It was probably this that caused Becket to change so radically in character in order to prove his devotion to the church. Henry, who thought he had an ally within the church who would help him in ecclesiatical disputes which had so plagued past kings, found he had an unpredictable opponent. Henry's short temper did not allow this to last for long and matters came to a head over the issue of clergy who broke the law. Henry maintained at a council held in October 1163 that these "criminous clerks" should be unfrocked and tried in a lay court. Becket maintained that they would be tried by ecclesiastical courts. Henry appealed to the new pope (Alexander III), who requested that Becket be more conciliatory. Henry now presented Becket with a series of terms, known as the Constitutions of Clarendon, which was where the council was held in January 1164. Becket argued tenaciously but eventually submitted. Henry believed he had succeeded but, soon after, Becket repented his change of heart and began lobbying the bishops. Henry was furious. He summoned him on various charges, including a debt of 44,000 marks (about £30,000) as owing since his days as chancellor. Becket was found guilty and his estates forfeited. He fled to France where he spent two years at the Cistercian abbey of Pontigny in Burgundy before the pope gave due attention to his cause. Becket pleaded personally before him in Rome, and Alexander restored him to the see of Canterbury. But Becket could still not return to England. He remained in France where he wrote letters of exhortation to the bishops, threatening excommunication unless they heeded his words.

In the meantime Henry had more pressing matters in hand. The pope's support and the Clarendon verdict had allowed Henry to start breaking down the old feudal system in England by ensuring that local baronial courts were subordinate to a strong central court. He re-established the jury system and introduced a new code of laws.

By a series of dynastic marriages Henry was establishing himself as one of the most powerful men in Europe. Already in 1160 he had arranged a marriage between his eldest surviving son, Henry, and Margaret, the daughter of Louis VII of France. Margaret was only two and Henry five, and Louis had not expected a confirmed marriage for many years after the betrothal agreement. But Henry had offered his support to the new pope, Alexander III, in 1160, whose succession was disputed, and in repayment, Alexander carried out the marriage. In February 1168 his eldest daughter Matilda was married to Heinrich the Lion, Duke of Saxony and Bavaria, whilst his youngest daughters were betrothed to the kings of Castile and Sicily.

Henry's dispute with Becket returned to haunt him in 1170. In that year Henry determined to have his eldest son formally crowned as king of England, which effectively elevated Henry himself into an imperial role. He needed the support of Becket and the pope in this, and begrudgingly accepted a reconciliation with Becket. However before this was fully resolved, Henry went ahead and had his son crowned (see Henry the young king) by the archbishop of York. Becket condemned this when he returned to England later that year. Becket was welcomed by the general populace as a hero: their champion against baronial oppression. Henry could not understand why Becket was always so quarrelsome. It was during one such moment of frustration that Henry uttered his notorious words: "Is there none will rid me of this turbulent priest?" Four knights, hearing these words and determined to prove themselves, immediately left Henry's court in Normandy, arriving at Canterbury on 29 December 1170 where they slew Becket within the cathedral. Although the murder shocked Christendom, it had not been at Henry's direct bidding. The knights each did their penance. Henry donned sackcloth, and apologised to the pope, but he soon weathered the storm. Everyone realised that Becket was best out of the way, though he was rapidly canonized in 1173.