In a few months, we will be commemorating the 800th anniversary of the sealing of Magna Carta—commemorating, but not celebrating; rather, mourning the blows it has suffered.

The first authoritative scholarly edition of Magna Carta was published by the eminent jurist William Blackstone in 1759. It was no easy task. As he wrote, “the body of the charter has been unfortunately gnawn by rats”—a comment that carries grim symbolism today, as we take up the task the rats left unfinished.

Blackstone’s edition actually includes two charters: the Great Charter and the Charter of the Forest. The former is generally regarded as the foundation of Anglo-American law—in Winston Churchill’s words, referring to its reaffirmation by Parliament in 1628, “the charter of every self-respecting man at any time in any land.” The Great Charter held that “No freeman shall be arrested or imprisoned,” or otherwise harmed, “except by the lawful judgment of his equals and according to the law of the land,” the essential sense of the doctrine of “presumption of innocence.”

To be sure, the reach of the charter was limited. Nevertheless, as Eric Kasper observes in a scholarly review, “What began as a relatively small check on the arbitrary power of King John eventually led to succeeding generations finding ever more rights in Magna Carta and Article 39. In this sense, Magna Carta is a key point in a long development of the protection of rights against arbitrary executive power.”

Crossing the Atlantic, the Great Charter was enshrined in the US Constitution as the promise that “no person shall…be deprived of life, liberty, or property, without due process of law” and that “In all criminal prosecutions, the accused shall enjoy the right to a speedy and public trial, by an impartial jury.” ...