The word treason, as transmitted to the English language from the Latin through the French, means “giving or delivering up.” The common law understood treason as treachery or breach of faith. It was therefore a crime committed between parties who enjoyed an established relationship of mutual benefit and trust. Petit treason referred to a wife’s killing her husband, or a servant’s or ecclesiastic’s killing his lord or master. High treason involved a breach between subject and sovereign, a betrayal of (or neglect of duty or renunciation of allegiance to, in word or deed) a sovereign to whom a subject owed allegiance by birth or residence. Sir Edward Coke, Baron de Montesquieu, Sir Matthew Hale, and Sir William Blackstone considered treason the highest of crimes and declared that it must be precisely defined to prevent its abuse by governmental authorities. In England, commencing during the reign of Edward III, Parliament narrowed the definition of treason but later widened it according to political exigencies.

The laws of the American colonies reflected the broad outlines of the common law of England, both as to breadth of the offense and severity of punishment, though sometimes the definitions of treason in the colonies were broader than the definition in England. By the eighteenth century, laws began more consistently to reflect the English law of treason, and eventually, during the Revolutionary period, came to require more precise definitions, more exacting standards of proof, and more lenient punishments. During the Revolution, many states adopted language recommended by the Continental Congress and its “Committee on Spies,” defining treason as adherence to the king of Great Britain (including accepting commissions from him) or to other “Enemies,” giving them “Aid and Comfort.”

Reflecting the American Founders’ concern with protecting individual rights and their fear of arbitrary governmental power, the Framers of the Constitution sought a precise and permanent definition of treason, the permissible means of proving it, and the limitations on the punishment for it. The drafters of the Constitution reached back (as had the Continental Congress) to language in the Statute of Treasons, 25 Edw. 3, 1351, ch. 2, stat. 5, which limited treason, among other things, to compassing or imagining the death of the king, levying war against the king, or adhering to the king’s enemies, giving them aid and comfort. But the Framers’ definition was even narrower. It did not include the language of “compassing or imagining,” which had been the basis of the English doctrine of “constructive treason,” an effective and easily abused method for dealing with political opponents. Thus, in the Constitution, treason consists only in levying war against the United States or adhering to its enemies by giving them aid and comfort. It may be proved only by confession in open court or on the testimony of no fewer than two witnesses to the same overt act.

The debates in the Constitutional Convention show an awareness of English common law and legislative history. James Madison suggested that the proposed definition reported by the Committee of Detail—limiting treason to the levying of war and adherence to enemies—was imprudently narrow and would effectively disallow the wisdom of experience. Others, such as John Dickinson, argued in favor of narrow wording. In the end, the phrase “giving them aid and comfort” was added to restrict even further the definition of the crime, and evidentiary requirements were tightened by the addition of the phrase “overt act.” Furthermore, as James Wilson noted in his 1791 Lectures on Law, treason requires generalized grievances and aims against the United States or its government as a whole, rather than particularized, essentially private grievances or aims. Respecting the federal nature of the union, the constitutional definition leaves open the possibility of concurrent state laws for treasons against them in their respective sovereign capacities.

When it came time to defend the Constitution, Madison left behind his earlier aversion to a narrow definition of treason and, in The Federalist No. 43, lauded the Convention’s wisdom as raising a constitutional bar to “new-fangled and artificial treasons” (understood as the results and instruments of faction), and as limiting the consequences of guilt. In The Federalist No. 84, Alexander Hamilton mentions the definition of treason as one of the guarantors of rights that make a separate bill of rights unnecessary.

The Supreme Court has had occasion to pronounce on treason, albeit infrequently. In Ex parte Bollman (1807), Chief Justice John Marshall rejected the idea of “constructive treason” and held that for treason to be established on the ground of levying war against the United States, an accused must be part of an actual assemblage of men for a treasonable purpose. Conspiracy short of the actual levying of war is insufficient. But in the related case of United States v. Burr (1807), Marshall tacked slightly. He again rejected constructive treason, but did so by holding that Aaron Burr, if not physically present in an assemblage of men, could still be convicted of treason on the testimony of two witnesses that he actively helped effect or aid such an assemblage—in effect, aided in the levying of war. Together, these cases made a treason conviction exceedingly difficult for anything other than manifest participation in a treasonable act.

After Burr, the leading treason cases grew out of World War II, for adherence to enemies. In Cramer v. United States (1945), the Supreme Court held that a specific intent—adherence to the enemy, and therefore to harm the United States— is necessary, rather than the simple rendition of aid. Further, the majority came close to holding that such adherence requires proof, not just of an act that on its face is “commonplace” (such as a meeting) but a manifestly treasonable overt act, evidenced by the testimony of at least two witnesses. But in Haupt v. United States (1947)—the Court’s first affirmation of a treason conviction— the Court effectively relaxed the standard of proof in Cramer by holding that the testimony of two witnesses to overt acts might be supported by other evidence as to the accused’s treasonable intent, including out-of-court confessions and admissions. In a concurring opinion, Justice William O. Douglas (who dissented in Cramer) affirmed that the separate elements of intent and overt act are amenable to different modes of proof, and only the latter triggers the constitutional requirement of testimony by two witnesses.

In Kawakita v. United States (1952), the Supreme Court held that dual citizenship does not diminish a citizen’s allegiance to the United States, and, in a treason prosecution, whether someone intends to renounce American citizenship hinges on particular facts and may be a question for a jury.

Lower courts have had occasion to enter verdicts of treason, commencing with the Whiskey Rebellion, some of them arguably on broader grounds than what the Supreme Court would later countenance. For example, courts held that armed resistance to the collection of taxes constituted constructive treason. A number of cases arising out of the Civil War also suggested, without directly interpreting the Constitution, that Confederate activities amounted to treason (although the general amnesty of December 25, 1868, pardoned all Confederates). Because of the particular and high constitutional standards associated with the definition and proof of treason, hostile or subversive acts falling short of treason but directed toward the whole polity have been prosecuted under various laws of Congress, including those dealing with espionage (for example, the conviction and execution of Ethel and Julius Rosenberg in 1953) and, more recently, terrorism. The exercise of federal prosecutorial discretion has also led to the prosecution on other grounds of individuals for acts that arguably amount to treason (for example, John Walker Lindh captured in Afghanistan in 2001), or to failure to prosecute at all.

In 2006, Adam Yahiye Gadahn—who, like Lindh, was a convert to Islam—became the first American to be charged with treason since Tomoya Kawakita in 1952. He remains at large but is accused of treason by knowingly adhering to and giving aid and comfort to an enemy of the United States by appearing in al Qaeda videos threatening Americans and the United States with violence. He is also accused of aiding and abetting al Qaeda through the provision of material support and resources.