The Magna Carta was the very first form of what we call a constitution today, wherein a formal document, signed by the king, stated that the king had to follow the laws of the land, recognizing that the citizens had rights, too. The Magna Carta was signed by King John on 19 June 1215 and is said to have inspired the American Revolutionists to fight against the British crown an also paved the way for trial by jury.
Conflict with the Church
In the early 13th century, King John, who was a very controversial figure, ruled over England and tried to claim supreme authority over European sovereigns. After he opposed the appointment of Stephen Langton as archbishop of Canterbury for the Roman Catholic Church, King John was excommunicated by the Pope. This led him to take revenge and tax the Church and also seize some of its properties.
Conflict with the Barons
The English barons also despised King John, who, after some horrible military decisions that led to defeat, taxed the English nobility to make up for his losses. When he, once again, tried to invade France unsuccessfully and expected the barons to fund him, they revolted.
Instating the Magna Carta
The archbishop of Canterbury organized a meeting with the barons and together they drafted the Magna Carta (then called the Articles of Barons). King John only affixed his seal to the document because he was afraid that, should he refuse, the rebellion would escalate in war. The Pope Innocent III however, after reconciling with King John, voided the Magna Carta only three weeks later. After King John died 1216 and the 9 year old King Henry III took the throne, the Magna Carta was reinstated. Since then, it has seen several revisions in 1216, 1217 and 1225