Whoever, owing allegiance to the United States, levies war against them or adheres to their enemies, giving them aid and comfort within the United States or elsewhere, is guilty of treason and shall suffer death, or shall be imprisoned not less than five years and fined under this title but not less than $10,000; and shall be incapable of holding any office under the United States. —18 U.S.C. 2381

The Justice Department’s announcement that it has obtained an indictment for treason against Adam Gadahn ups the ante on the legal front of the war against terror. The move is a long overdue warning for any frivolous American youths who are tempted to join the war against their country. Treason is not child’s play. It’s a deadly serious matter for the country, the traitor’s fellow citizens, and, now, for the traitor himself.

The definition of treason has changed over the centuries, but it has always been intended to include that species of crime that destabilizes not just a family or a neighborhood or even a city but the entire country. One of the earliest treason statutes in the English tradition was enacted during the reign of Edward III and proscribed, among other things, levying war against the king and adhering to and aiding and comforting his foreign enemies, precepts that found their way virtually unchanged into the American Constitution.

By its nature, there has always been a political nature to treason, and its prosecution at times became highly politicized in English history. Even so, revulsion at treason could “cross party lines.” William Blackstone recounted in his 1769 commentary on English law that Edward IV had prosecuted treasons committed against his predecessor, Henry VI, during the Wars of the Roses even though Edward IV denied the legitimacy of Henry VI’s claim to the throne.

Blackstone noted that treason statutes considered only the actual occupant of the throne even if someone else technically had a better claim and deserved to be king instead. Although the concept of a “loyal opposition” wouldn’t develop in its modern form until not long before Blackstone’s own day, Englishmen had long held that, to the extent dissent was permissible, it should only be voiced in the context of the normal political process and not in armed rebellion. Treason is very different from mere dissent.

The founders of America grasped the way in which treason was subject to abuse. So they prohibited Congress from defining it in any way other than levying war against the United States or adhering to its enemies, giving them aid and comfort. They prohibited the authorities from taking a confession anywhere but in open court and, absent that, prohibited the courts from bringing in a conviction save on the testimony of two witnesses to an overt act, and they insisted that the two witnesses had to have seen the same overt act.

Defining treason in the Constitution serves not only to entrench protections against abusive prosecution but to recognize the gravity of the crime. Owing to the high constitutional bar set for its prosecution, treason cases have been exceedingly rare in American history. As prosecutors noted in announcing the charges against Mr. Gadahn, the last such prosecution dates from the World War II era.

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So what does it mean that the Bush administration is now pursuing such a serious charge against an American? Our hope is that the administration seeks to underscore the fact that, de jure as well as de facto, America is at war. That this is a serious war and that in the midst of our raucous political debate lines exist which one cannot cross. This is why these columns have resisted the use of the word traitor to describe critics of the administration or to describe, say, those who leak confidential documents to the newspapers or the newspapers that print them. None of that is anywhere near the only definition of treason permitted in America.

But that doesn’t mean treason can’t be committed at a time when Al Qaeda is levying a war. Mr. Gadahn has for some time now, the indictment alleges, been a member of Al Qaeda and has gone so far as to appear in terrorist propaganda videos explicitly intended to intimidate all his fellow American citizens. Among the examples, as recounted in the indictment: “September 11th … notified America that it’s going to have to pay for its crimes and pay dearly”; “People of America … you too shall pay the price for the blood that has been spilled….”; and, “The streets of America shall run red with blood … casualties will be too many to count and the next wave of attacks may come at any moment.”

Such propagandizing has counted as “aid and comfort” in other treason trials in countries adhering to English common law. Indeed, one of the most famous treason prosecutions in the modern era was that of William Joyce, a British fascist known as “Lord Haw-Haw” who, after fleeing to Germany during the war and broadcasting Nazi propaganda across the Channel, became, in 1946, the last person executed for treason in Britain. By bringing Haw-Haw to the gallows, Britain established, at least for its law, that broadcasting propaganda is covered under classical treason.

Mr. Gadahn himself appears to represent a dangerous new class of Al Qaeda warrior — disaffected American youths who stumble into Islamic terrorism and along the way convince themselves of the worthlessness of their native society. Mr. Gadahn, according to press reports, started out life as a California teenager growing up on a goat farm with a penchant for heavy metal rock music and a messy bedroom. He discovered Islam via Internet chat rooms.

We’d like to think the prospect that following a spiritual quest into a terrorist organization could lead to the death penalty might discourage the Gadahns — and the John Walker Lindhs — of this world from starting down that road in the first place. It is something for parents, and for all Americans, to think about as they seek to explain the meaning of this war to their children. Codifying the understanding of treason common at the time of America’s founding, Congress applied the statute quoted above to those “owing allegiance to the United States.” That debt will not be easy to shed for any who have enjoyed the blessings of this land or who have plighted their allegiance to it.