The war did not end with Montfort's death, and Edward continued campaigning. At Christmas, he came to terms with the younger Simon de Montfort and his associates at the Isle of Axholme in Lincolnshire. In March he led a successful assault on the Cinque Ports. A contingent of rebels held out in the virtually impregnable Kenilworth Castle and did not surrender until the drafting of the conciliatory Dictum of Kenilworth. In April it seemed as if Gloucester would take up the cause of the reform movement, and civil war would resume, but after a renegotiation of the terms of the Dictum of Kenilworth, the parties came an agreement. Edward, however, was little involved in the settlement negotiations following the wars; at this point his main focus was on planning his upcoming crusade.
Edward took the crusader's cross in an elaborate ceremony on 24 June 1268, with his brother Edmund and cousin Henry of Almain. Among others who committed themselves to the Ninth Crusade were some of Edward's former adversaries. There was great difficulty raising funds for the expedition.
Originally, the Crusaders intended to relieve the beleaguered Christian stronghold of Acre, but before they could do this, several disasters happened to the French. The French forces were struck by an epidemic which, on 25 August, took the life of King Louis himself in 1270 By the time Edward arrived at Tunis, Charles had already signed a treaty with the emir, and there was little else to do but return to Sicily. The crusade was postponed until next spring, but a devastating storm off the coast of Sicily dissuaded Charles of Anjou and Louis's successor Philip III from any further campaigning.
Edward decided to continue alone, and on 9 May 1271 he finally landed at Acre. By then, the situation in the Holy Land was a precarious one. Jerusalem had fallen in 1244, and Acre was now the centre of the Christian area. The Muslim states were on the offensive under the Mamluk leadership of Baibars, and were now threatening Acre itself. An embassy to the Mongols helped bring about an attack on Aleppo in the north, which helped to distract Baibar's forces.
In November, Edward led a raid on Qaqun, which could have served as a bridgehead to Jerusalem, but both the Mongol invasion and the attack on Qaqun failed. Things now seemed increasingly desperate. Finally, an attack by a Muslim assassin in June forced him to abandon any further campaigning. Although he managed to kill the assassin, he was struck in the arm by a dagger feared to be poisoned, and became severely weakened over the following months.
It was not until 24 September that Edward left Acre. Arriving in Sicily, he was met with the news that his father had died on 16 November. Edward was deeply saddened by this news, but rather than hurrying home at once, he made a leisurely journey northwards. The political situation in England was stable after the mid-century upheavals, and Edward was proclaimed king at his father's death, rather than at his own coronation, as had until then been customary. In Edward's absence, the country was governed by a royal council, led by Robert Burnell. The new king embarked on an overland journey through Italy and France, where among other things he visited the pope in Rome and suppressed a rebellion in Gascony. Only on 2 August 1274 did he return to England, and was crowned on 19 August.
Edward's reign had two main phases. The first phase was administration of a now peaceful country. The second phase was warfare against Wales and Scotland.
His first concern was to restore order and re-establish royal authority after the disastrous reign of his father. To do this, he changed the administrators. He appointed Robert Burnell as chancellor, who held the post until his death in 1292. Edward then replaced most local officials, such as the sheriffs. This was done to prepare for an inquiry which would hear complaints about abuse of power by royal officers. Laws were made to define rights about ownership of land, recovery of debts, trade and local peacekeeping.
Edward reformed English Parliament and made it a source for generating revenue. Edward held Parliament regularly in his reign. In 1295 a significant change occurred. For this Parliament, in addition to the lords, two knights from each county and two representatives from each borough were summoned. Before, the commons had been expected simply to assent (say 'yes') to decisions already made by the rulers. Now they would meet with the full authority ("plena potestas") of their communities, to give assent to decisions made in Parliament. The king now had full backing for collecting 'lay subsidies' from the entire population. Lay subsidies were taxes collected at a certain fraction of the moveable property of all laymen. Historians have called this the "Model Parliament".
Llywelyn ap Gruffudd was the main Welsh leader. He refused to do homage to Edward, and married Eleanor, the daughter of Simon de Montfort. In November 1276, war was declared. Initial operations were launched under the command of Mortimer, Edmund Crouchback (Edward's brother) and the Earl of Warwick. Support for Llywelyn was weak amongst the Welsh.
In July 1277 Edward invaded with a force of 15,500 of whom 9,000 were Welshmen. The campaign never came to a major battle, and Llywelyn soon realised he had no choice but to surrender. By the Treaty of Aberconwy in November 1277, he was left only with the land of Gwynedd, but he was allowed to keep the title of Prince of Wales.
When war broke out again in 1282, it was entirely different. For the Welsh, this war was over national freedom. It had wide support, especially after attempts to impose English law on Welsh subjects. For Edward, it became a war of conquest. The war started with a rebellion by Dafydd (Llywelyn's younger brother), who was annoyed with the reward he had received from Edward in 1277. Llywelyn and other Welsh chieftains soon joined in, and initially the Welsh experienced military success. The Welsh advances ended on 11 December, however, when Llywelyn was lured into a trap and killed at the Battle of Orewin Bridge. The English conquest was complete with the capture in June 1283 of Dafydd, who was taken to Shrewsbury and executed as a traitor next autumn.
Further rebellions occurred in 1287–8 and in 1294. In both cases the rebellions were put down. By the 1284 law called the Statute of Rhuddlan, Wales was incorporated into England and was given an governing system like the English, with counties policed by sheriffs.
English law was introduced in criminal cases, though the Welsh were allowed to maintain their own laws in some cases of property disputes. After 1277, and increasingly after 1283, Edward embarked on a full-scale project of English settlement of Wales. He created new towns like Flint, Aberystwyth, and Rhuddlan.
Edward started a big program of building castles, to keep the Welsh under control. His castles started the widespread use of arrowslits in castle walls across Europe, drawing on Eastern influences. Also a product of the Crusades was the introduction of the concentric castle, and four of the eight castles Edward founded in Wales followed this design.
In 1284, King Edward's son Edward the later Edward II was born at Caernarfon Castle. In 1301 at Lincoln, the young Edward became the first English prince to be invested with the title of Prince of Wales.
Scotland and England were at peace in the 1280s. Alexander III of Scotland and Edward had an understanding whereby Alexander held land in England. This gave him the excuse to acknowledge Edward as his lord, and left ambiguous whether or not this applied to Scotland as well.
The heir to the throne was his infant granddaughter Margaret. Unfortunately, Alexander died in 1286, followed by Margaret in 1290. This left Scotland without a king, which started all the problems.