became King of England in 1199, but it was in troubled times. King Philip of France was continuing his attacks on Normandy, and was winning. This worried the great landowners - most of whom had lands in England and Normandy.
In that same, year, Arthur of Brittany was stabbed to death. He was John's nephew and a possible (but unlikely) claimant to the throne. Most people assumed that John was the murderer (in design, if not the deed). This lost John much of the loyalty due to the crown.
The biggest blow was the final loss of Normandy to France - in 1204. This wasn't just a matter of prestige, or even just wealth (although both were extremely important). The problem was those landowners with land on both sides of the Channel; they could only hold those lands if they swore exclusive allegiance to their lord - John in England, Philip in Normandy. Some managed to offload their Norman lands to relatives (in the hope that the family would bring the holdings together again in better times), some swaps were arranged - but many great men found their wealth halved at a stroke. (Oddly, William Marshal was one of the few who managed to get away with swearing allegiance to both Kings - even sending knights to both sides of a battle.)
The same problem affected John. His Norman lands were now gone (hence his nickname "Lackland"). Along with it went much of his power.
Things stumbled along for a few years, but in 1212, some of the Northern magnates conspired with Philip of France to get rid of John, and place Philip's son Louis on the throne of England.
John fought back (with the help of William Marshal), and had some success at first. But a major defeat in Normandy sealed John's fate; he had to give in.
The instrument of John's surrender was Magna Carta (the Great Charter), presented to the Barons at Runnymede on the 15th of June, 1215.
In the modern world, we sometimes characterise Magna Carta as a great reforming charter establishing and protecting the rights of the common man against over-mighty governments. It was nothing of the sort, at the time. It was a document of surrender to specific grievances, assuring the power of the Barons - the "common man" wasn't on anyone's mind. (However, some commentators reckon that clever John had pulled a fast one on the Barons by couching his concessions in general terms which would rebound on them, when their underlings realised that it could apply to them, too.)
As a surrender, Magna Carta was a dismal failure. Many Barons rejected it outright, and the war against John continued. By this time, Philip's son Louis was ensconced in London, waiting for John to fall or be captured, and for an invasion from France, at which point he would take the English crown, and unite it with the French.
John fought back, as best he could (again, with the help of William Marshal). He repudiated Magna Carta and attacked the rebels again - with some success. But the King fell ill and died quickly, begging forgiveness for his sins.
John's death seemed to wipe the slate clean - as far as the English Barons were concerned. They weren't particularly keen to be ruled by a Frenchman, and couldn't justify visiting John's sins on his son.
Throughout this tumult, Henry had been kept safe at Devizes by William Marshal (whose extensive West Country lands now contained the castle).
After John's funeral, Henry was summoned from Devizes to meet his subjects at Gloucester and was crowned King within ten days of his father's death. Marshal was appointed Regent for the underage King.
It took another couple of years to get rid of Louis, and the threat of French invasion. Again, Marshal was the general who achieved this.