There are few documents in Anglo-American constitutional history more sanctified than the Magna Carta. On its face, the document is a medieval charter commonly used to convey land, authorized in 1215 by King John of England (1167–1216) under threat of civil war from his barons. Since that date, significant constitutional principles have been attributed to the Magna Carta, even though it was a document drawn up in the midst of a violent political crisis by selfish men gifted neither with prescience nor political genius. Yet for many centuries historians and politicians have celebrated the date of its creation, and the venerable American Bar Association has erected a monument in England in its honor. Understanding the document's true importance depends on appreciating its historical context and limitations.

The nobles and others who met with King John and secured his agreement to sign the Magna Carta on June 15, 1215, were desperate...