In a last-ditch effort to quash an armed rebellion against him, King John of England signed a document called the Magna Carta. This document basically was him promising to stop being a jerkface all the time—you know, doing things like claiming all the good forests for himself and coming up with new taxes every ten minutes.
As a peace treaty the Magna Carta failed miserably, mainly due to the fact that King John never had any intention of doing anything that it said. However, it did contain some seriously ahead-of-their-time ideas about fair trials and limited government. And that inspired a lot of other people to create systems where leaders were actually prevented from being jerkfaces.
So, thanks to King John's attempt to weasel out of a war he couldn't win, the world is a better place.
The ideals of the American Revolution and the creation of the U.S. Constitution both owe a great debt to the contents of the Magna Carta, which was the first document to offer civil liberties.
The signing of the Magna Carta was the beginning of the end for the divine right of kings and absolute monarchies all over the world because it started a chain reaction that continues to present day.
A bunch of rich dudes with swords got mad at their king for the super annoying way that he did whatever the heck he wanted to all the time. After a tense meeting in a soggy meadow, they all agreed to a plan that would limit the king's power.
First—and most importantly for the people who wrote it—the Magna Carta was designed to get the barons and the king to leave that soggy meadow without slicing each other up with their big, sharp swords. It contains several clauses about the release of prisoners and about everybody going home happy and with all their limbs intact.
Second, the Magna Carta was a list of grievances that could easily have been titled, "Things We Hate About King John." It's a long list, but most of the complaints come down to the fact that King John was abusing his ability to make up new taxes, claiming pieces of the English countryside for himself, and punishing people without letting them stand trial. A lot of the clauses are King John promising not to pull those sorts of shenanigans anymore in every detailed way that the barons can think of.
Finally, the Magna Carta ends with some bonus clauses clearly written by an overachieving baron angling for some extra credit. It specifies that to enforce all the other clauses there needs to be a council of 25 barons to basically keep King John (who they still don't trust) in line.
An unpopular king accidentally agrees to forever improve the world's governments.