Magna Carta
In 1200, King John became involved in a long-drawn out war with France. This war was expensive and John was forced to introduce new taxes to pay for his army. This created a great deal of resentment in England, and John's position was not helped when, in 1205, the king's army lost control of Normandy, Brittany, Anjou and Maine.
In 1215, King John made another desperate attempt to gain control of his lost territory in France. Once again he was defeated and was forced to pay £40,000 to obtain a truce. When John tried to obtain this money by imposing yet another tax, the barons rebelled. Richard of Clare, third Earl of Hertford, was one of the leaders of the rebellion. Few barons remained loyal, and in most areas of the country, John had very little support. John had no chance of victory and on 15th June, 1215, at Runnymede in Surrey, he was forced to accept the peace terms of those who had successfully fought against him.
The document the king was obliged to sign was the Magna Carta. In this charter the king made a long list of promises, including no new taxes without the support of his barons, a reduction in the power of his sheriffs and the right of a fair trial for all freemen.
The barons had doubts whether King John could be trusted to keep his word. A small group of barons were given the task of making sure that John kept the promises he had made in the Magna Carta. Two of the barons chosen were Richard of Clare and his son Gilbert de Clare.
Soon after he signed the charter the king appealed to Pope Innocent III for help. The pope was concerned about this rebellion and decided to excommunicate the barons who had fought against their king. The pope also provided money to help King John recruit foreign mercenaries to fight against his disloyal barons. The civil war resumed. One of King John's main targets was Richard of Clare and in November, 1215, his troops seized his castle at Tonbridge.
The following year King John died and was succeeded by his son Henry III. A year later Richard de Clare also died. Although the sons of bitter enemies, Henry III and Gilbert de Clare became close friends and it was not long before the family were given back Tonbridge Castle.
Gilbert de Clare was now the most powerful magnate in England. He controlled 456 manors and when requested, had to supply the king with 260 knights. In 1230, Gilbert de Clare agreed to help Henry III win back land that King John had lost in France. The following year Gilbert was killed while fighting in the king's army.

Primary Sources
(1) Extracts from the Magna Carta (1215)
(I) In the first place we have granted to God... that the English Church shall be free... freedom of elections, which is reckoned most important and very essential to the English Church...
(II) If any of our earls, or barons... shall have died, and at the time of his death his heir shall be of full age... he shall have his inheritance...
(VII) A widow, after the death of her husband, shall without difficulty have her inheritance...
(VIII) No widow shall be compelled to marry, so long as she prefers to live without a husband...
(XI) If anyone die indebted to the Jews, his wife shall... pay nothing of that debt.
(XII) No scutage or aid (tax) shall be imposed on our kingdom, unless by common counsel of our kingdom...
(XIV) And for obtaining the common counsel of the kingdom before the assessing of an aid or of a scutage, we will cause to be summoned the archbishops, bishops, abbots, earls, and greater barons...
(XX) A freeman shall not be fined for a slight offence... and for a grave offence he shall be fined in accordance with the gravity of the offence... and a villein should be fined in the same way.
(XXIII) No village or individual shall be compelled to make bridges or river banks...
(XXX) No sheriff or bailiff... or other person, shall take the horses or carts of any freeman for transport duty, against the will of the said freeman...
(XXXV) Let there be one measure of wine throughout the whole kingdom, and one measure of ale; and one measure of corn; and one width of cloth...
(XXXIX) No freeman shall be taken or imprisoned or outlawed or exiled or in anyway destroyed... except by the lawful judgement of his peers or by the law of the land.
(XL) To no one will we sell, to no one will we refuse justice.
(XLI) All merchants shall have safe and secure exit from England, and entry to England, with right to be there and to move about... for buying and selling... except in time of war, such merchants as are of the land at war with us. And if they are found in our land at the beginning of the war, they shall be detained, without injury to their bodies or goods, until information be received by us by our chief justicar how the merchants of our land found in the land at war with us are treated; and if our men are safe there the others shall be safe in our land.
(XLII) It shall be lawful in future for anyone to leave our kingdom... excepting those imprisoned or outlawed in accordance with the law in the kingdom...
(XLV) We will appoint as justices, constables, sheriffs, or bailiffs only such as know the law of the kingdom and mean to observe it well.