Pretty much every fixed idea in history goes through a period of revisionism, and the more you read into the past the more you realise there are two sides to the story and everything is a blur.

Even in the case of King John, considered England's worst monarch, there was an attempt to do this in recent years.

It didn’t get very far; all the evidence suggests that he was in every way a terrible, terrible man. Yet, despite or perhaps because of this, he did more than any other king to introduce a system of rights into English law.

Gerald of Wales called John “a tyrannous whelp”; another chronicler, William of Newburgh, said he was “nature’s enemy”. Even one of John’s own generals, who was on his payroll, conceded that he was “a very bad man, cruel and lecherous”. The kindest word comes from twentieth-century historian RV Turner, who wrote that “compared with Hitler and Stalin... John seems quite tame” – not exactly a ringing endorsement.

John was the youngest of eight children of Henry II and Eleanor of Aquitaine, and from a young age was a boisterous boy; in one charming episode from his youth he lost his temper while playing chess, which at the time was played with enormous, heavy pieces, and smashed his opponent over the head. With his father’s huge empire already parcelled up between his three surviving elder brothers, John was made teenaged despot of Ireland as a consolation, but during his first visit in 1185 he annoyed the native aristocrats by pulling their beards, then squandered all his soldiers’ pay, much of it on prostitutes and drink, and by the time he was ordered home had managed to achieve the impossible by uniting English soldiers and Irish natives in hating him.

His brother, the heroic crusader Richard I, had been nicknamed “Richard yay-or-nay” (that is, straight answer). But John ignored almost every oath he took; he had broken his father Henry II's heart by joining a rebellion against him, and while his brother was imprisoned in Germany he had conspired against his arch-enemy the king of France to take the throne.

He was also cruel and ruthless even compared to his contemporaries, which was a low bar. Most infamous, for some, was his execution of 28 sons of Welsh princes who had rebelled against him, or his murder of his own soldiers who did not know that he had changed sides in the war without telling them. He was also a notorious letch and drunk, and one of the probable causes of the conflict that led to Magna Carta was John's habit of trying to have sex with the wives and daughters of the nobility.

And he was – at least by modern-day standards – a paedophile.

Still, John was tender-hearted about animals and doted over his pet falcon, Gibbun, who was fed on doves, pork and chicken once a week. So, good in everyone then.

All of John’s family were cruel, yet unlike his father or brother, he was also a coward, and dismally bad at fighting; during his 17-year reign he levied tax after tax to wage war in France, a conflict he lost disastrously, and after his final, humiliating military defeat in 1214, unrest burst into the open and a group of rebel barons defied the king, renouncing homage and fealty. What emerged was Magna Carta, only a peace treaty at the time but which over the following century would become a firmly established part of English law.

The war with France had begun after John had grown bored with his first wife and decided to take another, Isabella of Angoulême, who apart from being a child and already engaged, was a perfect choice. Her fiancée Hugh “le Brun” of Lusignan appealed to his lord, King Philip of France, who ordered John to a meeting; the English king promised to do so but when he failed to appear, Philip declared him a “contumacious vassal” who had forfeited Aquitaine, Poitou and Anjou. Now French soldiers invaded Normandy, and Lusignan, and the other Poitevin barons were joined by Prince Arthur, John's young nephew who had a better claim to the throne.

John arrived in Normandy and invited his nephew around for talks in his castle; things obviously got out of hand and Arthur’s body was seen floating in the Seine a couple of days later. He might have jumped out of a tower, or John may have crushed Arthur’s head with a stone after flying off the handle in a drunken rage. Either way, it wasn’t a good week for Arthur.

However when Philip of France invaded Normandy, John simply fled. The following year John threatened to invade France with an “unbelievably large” force, and raised an invasion army at Portsmouth, but the entire fleet mutinied and he had to back down. Now tax went up by 300 per cent, a policy that unsurprisingly wasn’t very popular; he demanded “scuttage”, or shield tax, the payment of money by barons as a substitute for doing military service in defence of Normandy. Yet many Anglo-Norman aristocrats no longer held lands across the Channel, and saw little reason why they should bother helping.

John managed to balance the books for the next few years largely because he was excommunicated by the Pope for refusing his choice of Archbishop of Canterbury and threatening to kill the Holy Father. It was the medieval equivalent of a general strike, meaning no church services could be held in the country; but John was perfectly happy as it meant he collected the vast revenues from the English dioceses. (John was notoriously unreligious; during one Easter service, when St Hugh of Lincoln was giving the homily, John sent the bishop three notes telling him to hurry up so he could go to lunch. During Mass he was forced to attend he’d take out a gold coin at collection time, ostentatiously play with it, then put it back in his purse, Blackadder-style.)

John was a monster, and possibly unhinged. However in medieval times people would generally go along with the king, however mad or rapacious he was, as long as he kept winning battles, and unfortunately John wasn’t very good at this. The king decided to invade France, a scheme which in terms of relative financial expenditure rivalled that of 1066 or the Normandy Invasion of 1944; a massive enterprise, yet it all went terribly wrong.

To raise money, in January 1214 the king auctioned off his first wife to the baron Geoffrey de Mandeville for 20,000 marks, which Geoffrey couldn’t afford, and since she was too old to have kids he probably did not want the marriage. The army in France was led by the king’s half-brother, William Longsword, one of Henry II’s numerous bastards, who to start with did well in Flanders. (It says something about the old king’s rapacious sexual appetites that Longsword’s mother, countess of Norfolk, was the cousin of one of Henry II’s mistresses and the daughter of another.)

But things soon went badly and the enterprise culminated with disaster at Bouvines in July 1214, when John’s last hope of holding onto his continental empire ended. John’s grand alliance had failed, and pretty much ten years’ worth of money went up in smoke; Longsword was captured and, while he was in a French jail, John tried to have sex with his wife.

When John returned from France in October he was broke and his opponents were now swelling in number. In January 1215, in a last desperate bid to distract from his troubles, John announced he was going on crusade, and that anyone who followed him would get to wear a shiny white cross. There were no takers, but having taken the cross, or at least promised to, the king was immune from attack. That month the king and 40 barons met in London, agreeing to rendezvous again in April.

The barons demanded that the king obey Henry I’s Charter of Liberties, which had been issued a century earlier guaranteeing certain protections for the nobility, but John stalled and then double-crossed them, asking the pope to intervene. The “Northerners”, as the rebels were called, raised an army and headed south and on May 5 officially renounced their loyalty to the king, who had failed to show up for a meeting.

With the country on the verge of civil war, Archbishop of Canterbury Stephen Langton persuaded the two sides to agree a peace treaty, with a series of demands agreed to at a spot in Surrey called Runnymede on 15 June. However the king soon went back on his word and launched a war against the rebels, and was doing quite well when he contracted dysentery and died in October 1216. His nine-year-old son’s guardians, desperate to avoid defeat, reaffirmed the charter in 1217 and later, to distinguish it from other royal charters, it became known as the Great Charter, or Magna Carta, and it became formal law in 1297.

As for John, the gossipy Benedictine monk Matthew Paris famously summed up the nation's feelings with the pithy remark: “Hell herself felt defiled by his admission.” But history can be a funny thing; as Winston Churchill put it in The History of the English-Speaking Peoples, “When the long tally is added, it will be seen that the British nation and the English-speaking world owe far more to the vices of John than to the labours of virtuous sovereigns; for it was through the union of many forces against him that the most famous milestone of our rights and freedom was in fact set up.”

So each June 15, raise a glass to the hopelessly incompetent alcoholic king who gave us our freedom.

Ed West's 1215 and All That: A very, very short history of Magna Carta and King John, is available on Amazon.

This article is part of the New Statesman's Monarchy Week. Find more here.