After John's death, William Marshall, the new protector and Regent of England, recruited and reinforced the already powerful royal Anglo/Norman army, with mercenaries and the many friendly Anglo/Norman barons. He planned to rid England of the Dauphin and all these ambitious French undesirables. Bear in mind, they were French, not Norman as he was. Nominally a lawyer, he soon learned the military skills. Marshall ran a vigorous campaign against Prince Louis, the self-proclaimed King of England, Wales, Scotland and Ireland, and his French army who were still occupying London with our 'heroes', the traitorous Magna Carta Barons, now the acknowledged vassals of the Dauphin and the King of France.

In 1217, William Marshall took Saire de Quincey's Castle of Montserol. Lord Fitzwalter, the principal instigator of the barons rebellion, who was still in London, and had now elevated himself to "Marshall of the Army of God", presumably in competition with Pope Innocent, or at least, revealing his aspirations to powerful grandeur. Here is perhaps revealed Fitzwalter's real objectives. He raised an army of 600 knights and 20,000 French soldiers, and advanced on Marshall. Fitzwalter, and the Dauphin, the new King of England, and the Count of Perche his military commander, marched on Montserol. Marshall wisely retreated to Nottingham.

The Count of Perche, after finding Montserol empty but secure, moved his force to Lincoln Castle, another royal castle. Marshall, with reinforcements, now including his own son, William Marshall jr, and other apprehensive defectors from the Magna Carta barons who had departed from their surety, promptly advanced on Lincoln and laid siege. The French army and the traitorous Anglo/Norman barons were driven back into the Castle. The siege rampaged. Most of the Magna Carta Barons and 300 other knights were taken prisoner. There was slaughter enacted by Marshall's superior force, and much blood letting

From "Debrett's Kings and Queens of Britain" by David Williamson, ISBN 0-86350-101-X, p.69:
The regency was exercised by William Marshal, Earl of Pembroke, until his death in 1219, and then by Hubert de Burgh.

From "A New History of Ireland", v IX, p 174:
3d Lord of Leinster 1189-1219.

In a room of the Tower of London in August 1189, two people who were about to be married met for the first time. This twist of fate or act of destiny would have a far-reaching effect on English history. The young lady was Isabel de Clare, sole heiress of Richard Strongbow de Clare, Earl of Pembroke and Striguil, and Aoife, daughter of Dermot MacMurrough, King of Leinster. The man was William Marshal, the second son of John the Marshal and Sibyl, sister of Patrick, Earl of Salisbury. There are no accounts of this first meeting nor of their marriage ceremony, but this was the final step in the making of one of the greatest knights and magnates of medieval English history.
William Marshal's life is well documented because less than a year after his death in 1219, his eldest son William II commissioned a record of his father's life. "L' Historie de Guillaume le Marechal," is a metrical history of a man and of the knightly class in the late twelfth and early thirteenth century. William Marshal was born c 1146, and as a younger son, becoming a knight was his natural choice of a path to success and survival. Marshal was sent to his father's cousin William of Tancarville, hereditary Chamberlain of Normandy, to be trained as a knight in c1159. He was knighted, probably by his uncle, in 1167.
In 1170 William Marshal was appointed head of the mesne (military) household of the young Prince Henry by King Henry II. From this time until young Henry's death in June of 1183, Marshal was responsible for protecting, training and running the military household of the heir. In 1173, William Marshal knighted the young Henry, and thereby became Henry's lord in chivalry. We know that Marshal led young Henry and his mesne to many victories on the tournament fields of Normandy.
It is during the years from 1170 to 1183 that William Marshal established his status as an undefeated knight in tournaments. It is here that Marshal began to establish his friendships with the powerful and influential men of his day. His reputation and his character were built through his own actions and abilities. In this age of feudalism, Marshal was a landless knight. He had no lord from whom he could gain advantages or status.
On the death of the young Henry, Marshal obtained permission from Henry II to take the young Henry's cross to Jerusalem. Marshal spent two years in the Holy Land fighting for King Guy of Jerusalem and the Knights' Templar. There are no known records of his time in the east, but we know that some of the castle building techniques he later used at Pembroke were probably learned here.
Henry II granted Marshal his first fief, Cartmel in Lancashire, in 1187. With this fief Marshal became a vassal of King Henry II and swore fealty to him as his lord and his king. Until Henry II's death in 1188, William Marshal served as his knight, his counselor, and his ambassador. When Richard I came to the throne, he recognized Marshal as a brother and equal in chivalry. Fulfilling the promise made by his father, Richard gave Marshal the heiress Isabel de Clare and all her lands in marriage. With this marriage, William Marshal became "in right of his wife" one of the greatest lords and magnates in the Plantagenet kingdom. Isabel brought to Marshal the palatine lordships of Pembroke and Striguil in Wales and the lordship of Leinster in Ireland. These were large fiefs of land where the lord held as tenant-in-chief of the Crown. A palatine lord's word was law within his lands. He had the right to appoint his own officials, courts and sheriffs, and collect and keep the proceeds of his courts and governments. Except for ecclesiastical cases, the king's writ did not run in the palatinates.
King Richard also allowed Marshal to have 1/2 of the barony of Giffard for 2000 marks. This barony was split with Richard de Clare, Earl of Clare and Hertford, who held the barony in England as lord while Marshal held the land in Normandy as lord. This gave Marshal the demesne manors of Crendon in Buckinghamshire and Caversham in Oxfordshire, for 43 knights' fees, and the fief of Longueville in Normandy with the castles of Longueville and Mueller and Moulineaux, for about 40 knights' fees. Marshal considered the lands that he held to be one unit, not separate units of English, Irish, Welsh, and Norman lands. They were a compact whole to be preserved and improved for the inheritance of his children. Marshal used what he had learned fighting in Normandy and in the Holy Land to improve these fiefs. Without including his lands in Normandy and Ireland, as feudal lord Marshal controlled a vast amount of land, wealth, and knights/vassals in the Angevin kingdom. William Marshal served King Richard faithfully as knight, vassal, ambassador, itinerant justice, associate justiciar, counselor, and friend.
On Richard I's untimely death in 1199, William Marshal supported John as heir to the throne rather than John's nephew, Arthur of Brittany. It was King John who belted William Marshal and created him Earl of Pembroke on the same day that John was crowned King, May 27, 1199. It is during King John's reign that the character of William Marshal is clearly revealed. John's character has been drawn by countless historians, and none have been able to erase the ineptitude that King John displayed when dealing with his English barons. Whatever his motives were, John inevitably alienated his greatest barons despite the fact that he needed their support and loyalty to rule England.
William Marshal was a powerful, respected, wise and loyal knight and baron who had already served two Angevin kings. King John, however, accused Marshal of being a traitor, took all of Marshal's English and Welsh castles, took Marshal's two older sons as hostages, tried to take Marshal's lands in Leinster, and even tried to get his own household knights to challenge Marshal to trial by combat. Despite all of this, William Marshal remained loyal to his feudal lord. He did not rebel when John took his castles; he gave up his two sons as hostages; he supported John against the Papal Interdict; and he supported John in the baronial rebellion. Of all the bonds of feudalism, the greatest and the most important bond was the one of fealty, of loyalty to one's lord. To break this bond and oath was treason, and this was the greatest of crimes.
William Marshal was the epitome of knighthood and chivalry. He did not simply espouse it. Marshal's entire life was governed by his oaths of fealty and by his own innate sense of honor. If Marshal had taken his lands, castles, and knights to the side of the rebellion, King John would have lost his crown and perhaps his life.
On the death of John, October 19,1216, William Marshal was chosen by his peers in England as regent for the nine year old Henry III. Henry was knighted and then crowned under the seal of the Earl of Pembroke. William Marshal was the main force and impetus for the defeat of Philip II of France, even leading the attack to relieve Lincoln castle in May 1217 though he was seventy years old. On September 11, 1217, Marshal negotiated the Treaty of Lambeth that ended the war. By his wise treatment of those English barons who had supported Philip II against King John, Marshal ensured the restoration of peace and order in England. This undefeated knight had become a great statesman in the last years of his life. William Marshal died May 14, 1219 at Caversham and was buried as a Knight Templar in the Temple Church in London.

William Marshal had been born during the Civil Wars of King Stephen and Empress Mathilda. He trained and knighted one intended king; served faithfully Kings Henry II, Richard the Lionheart, and John Lackland; and knighted and served as regent for a fourth king. As "rector regis et regni," Marshal had the Great Charter reissued in 1216 and in 1217 for the welfare and future of England and the Crown. There are many explanations and definitions of Marshal, his life and his time. Some say he survived so long and so well because of his physical stamina and condition, that he was simply a man of great physical strength. This gives only a piece of the complete portrait of William Marshal. He was a brilliant strategist in terms of his world, militarily and politically. He lived and survived in Henry II's arena, earning Henry's respect and affection. No man of little intelligence would have survived very long there. William Marshal can be understood in terms of his world of feudalism, fealty, loyalty and honor. Marshal stood by King John because of Marshal's oath of fealty and homage to his "lord," who also happened to be the King.
William Marshal was a man who lived his life according to his sense of honor, and his sense of honor was defined in the laws and customs of feudalism and knighthood. It is that sense of honor that made no man equal to William Marshal, knight, Earl of Pembroke and Striguil, Lord of Leinster, and Regent of England. Of all the barons assembled at Runnemede, William Marshall was the most distinguished.  He was not one of those opposed to King John; he was an old man and had served as counselor to three Plantagenet Kings, His part was that of intermediary.  While not a surety he deserves our veneration more than any of the twenty-five. It was he who made possible the realization of Magna Carta. In his youth he was acclaimed by his contemporaries as the perfect type of chivalry.  In his old age and in history he appears as one of the noblest of medieval soldier statesmen.  He filled the foremost places in England and Ireland, and never compromised his honor. He is described as tall and handsome, of beautiful countenance and dignified bearing, with unusual strength.
We do not know the exact date of his birth but it must have been shortly before 1140, for as a boy in 1152 he was given by his father as a hostage to King Stephen, and he was over 80 when he died in 1219. He was the second son of John Marshall, by his wife Sybil, sister of Patrick, Earl of Salisbury, and daughter of Walter d’Evreux, and grandson of Gilbert Marshall from whom he inherited the office of Marshall. His early life reads like a story from King Arthur’s Knights of the Round Table.  He seemed to move in an aura of fantasy, as if led to perform legendary deeds.  He saved the life of Richard Coeur de Lion, carried the heart of young King Henry, son of Henry II, to Palestine, served as guardian to two boy Kings and remained unmarried until at the age of 50 he could win the hand of a great heiress, and then had ten children. In 1167 he returned to England to live with his uncle, the Earl of Salisbury. Shortly after he followed the Earl in a campaign in Normandy, where, while defending his uncle in an ambush he was taken prisoner, but later escaped.  At this time he was unsurpassed in the tourney; it is said that, with a companion, Roger de Gaugi, captured 103 Knights, in different parts of France.
In 1170 he was given charge of the young King Henry, son of Henry II, who was made, although a boy, King of England during his father’s life time.  Thus, early in his life, he was charged with loyalty to the crown, and remained constantly with the prince until the latter’s death in 1183.  The young Henry was so devoted to William that, on his deathbed, he beseeched his friend to take his heart to the Holy Land, which William Marshall did the same year. In 1188 a crucial event in his life occurred.  He was in battle fighting the forces of Prince Richard, who was in rebellion against his father, Henry II, when he came face to face with the prince and could have slain him.  Richard called out "slay me not, for I am unarmed." William killed Richard’s horse instead and saved the prince.
One year later William was at the deathbed of Henry II. On Richard’s accession to the throne, he remembered he owed his life to William, and gave him in marriage to the great heiress of Struguil, Eva, daughter of Richard de Clare, Earl of Pembroke and granddaughter of Dermot, King of Leinster.  She was only seventeen, he about fifty.
Soon after Richard left on the Crusade for which he became so famous; William Longchamps was left in charge of the kingdom, and William Marshall counselor to him. On Richard’s return he continued as advisor to the King, and acted as mediator between Richard and Philip of France.
At Richard’s death in 1199, William Marshall seems to have been the one responsible for John’s succession.  Hubert Walter, Archbishop of Canterbury, headed the council to decide between Arthur, who, as son of John’s elder brother Geoffrey Duke of Brittany, was therefor first in succession, and John himself.  In the discussion William exclaimed "Arthur is a tool of the French, he loves not England." He sensed that John, in spite of his trickery, would be accepted by the English. In this decision, we may not discern the wisdom for which William was noted, for our sympathies are apt to be swayed by romance.  But from the facts we know of Arthur he was a worthless fellow, not a boy at all as Shakespeare described him, but a young man with little sense of loyalty. He had never been in England.
  During most of John’s reign, Marshall was occupied with his estates in Ireland.   He sheltered there William de Braose who fled from Wales - a matter which brought John to Ireland.  Here, William’s firmness and reasonableness, combined with his reputation for loyalty to the crown, saved him from John’s revenge.  He always used his influence on the side of justice and moderation; John continued to employ him as mediator between himself and his barons. 
As we know already, he was at King John’s side at Runnemeade, although in sympathy with the baron’s cause.  What part he played in the framing of Magna Carta is a matter of conjecture, but it is difficult to believe that Stephen Langton was alone responsible, or that the justiciars, or William Hardell had the foresight and sagacity to word many of the clauses that made the Carta immortal.  William Marshall had those qualities, besides that serenity of mind that comes from age and experience.  His biographers, while unstinting in their admiration, point out he belonged to the age of feudalism, his own age, when loyalty to one’s overlord was the supreme virtue; he was no prophet of a new order.  On the other hand, Magna Carta was not a revolutionary document; it was eminently a practical body of laws and judgements based largely on previous and existing laws and customs.
William Marshall was the supreme man for the hour when John died, leaving as his heir a boy of nine. One half of the kingdom was in the hands of the French King; there was civil war.  To William Marshall was entrusted the task of guarding the safety of the young king, whom he personally  knighted; he was appointed "Guardian of the King and of the Realm." His direction of the battle of Lincoln was a masterpiece of strategy.  It is worth noting that Saher de Quincy was taken prisoner by him in this campaign. It was during the three years of disorder after the death of John that William Marshall’s greatest and special work was done accomplishing the pacification of the country.  Only his wisdom and courage preserved England from becoming a tributary province of France; it was he in truth who made possible the realization of Magna Carta.  
As death approached, he retired to his manor of Cavershamin Berkshire, which was his patrimony.  He had castles in Ireland and Wales where he could have died surrounded by pomp, but he preferred the place of his birth. He died May 14, 1219. As he had been in the Holy Land, he was buried in the Knight’s Templer’s Church in London, where his tomb still exists.