800 Years Ago

The Magna Carta was signed (June 15th in the Julian calendar). Its primary purpose was to end a civil war between the English barons and King John.

An argument can be made that the Magna Carta was pretty much ignored by the English kings until the English Civil War in the seventeenth century. The Magna Carta was resurrected as an argument against the divine right of kings, to show that limitations on the monarch's powers were not a new and radical idea, but part of English legal tradition. Its resurrection was still fresh when the United States was created.

Despite those who would assert that presidential power is unlimited*, that limitation derives from the Magna Carta.

Not too shabby for 800 years.
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* The two Despicable Richards, for example (Nixon and Cheney).