Of the 37 counts leveled at President Trump in Special Counsel Jack Smith’s indictment, 31 of them relate to the Espionage Act, telegraphing that the case against Mr. Trump is likely to double as a turn in the spotlight for a law that many dissidents and radicals have loathed for more than a century. Now, if a jury determines that Mr. Trump acted crosswise with its provisions, he could go to prison.  

The inclusion of the Espionage Act — actually a cluster of related statues — could not have come as a surprise, as it was mentioned in the warrant that swung open the gates of Mar-a-Lago to the FBI last August. It targets anyone who “unlawfully retains” control over “any document, writing, code book, signal book, sketch, photograph, photographic negative, blueprint, plan, map, model, instrument, appliance, or note relating to the national defense.” 

Mr. Smith’s indictment alleges that President Trump hoarded documents “regarding defense and weapons capabilities of both the United States and foreign countries” as well as information on the “United States nuclear program” and “plans for possible retaliation in response to a foreign attack.” After his presidency ended, Mr. Smith claims, Mr. Trump was “not authorized to possess or retain these classified documents.”

Mr. Smith asserts that “national security information was information owned by, produced by, produced for, and under control of the United States government.” A photograph in the indictment indicates that some of that information resided for a time in boxes stacked on a stage at Mar-a-Lago’s White and Gold Ballroom, where “events and gatherings took place.” A photograph indicates they were subsequently moved to a shower and bathroom. 

In a Saturday speech in Georgia, Mr. Trump said with respect to the charges that “you’ve got a box, I’ve got a box” and added that boxes of materials were “openly sitting on the White House sidewalk,” where people were taking pictures with them. The former president cited the Presidential Records Act as the “ruling law” in the case, and noted that it was not cited by Mr. Smith in the indictment.

The Espionage Act, which the Wall Street Journal calls “ancient and seldom-enforced,” was born during the First World War. Its penalties run from 10 years in prison to execution. The version in effect between 1918 and 1921, supplemented by the Sedition Act, went even further than its modern version, prohibiting “disloyal, profane, scurrilous, or abusive language” against the war effort.

In its original form, the Espionage Act, enacted two months after America entered World War I, targeted written material “urging treason.” Under the law, 74 newspapers were denied the privilege of using the United States Postal Service. Despite the law’s apparent affront to the First Amendment, it was repeatedly upheld by the Supreme Court. The Sedition Act was repealed in 1921.

The signal challenge to the Espionage Act was mounted in 1919 by a socialist, Charles Schenck. He distributed leaflets arguing that the military draft violated the Thirteenth Amendment’s interdiction of involuntary servitude. He was convicted of conspiracy to violate the Espionage Act, a law that he maintained violated the First Amendment. 

In Schenck v. United States, the high court unanimously  found for President Wilson’s expansive use of wartime powers. Justice Oliver Wendell Holmes articulated the “clear and present danger test,” whereby the First Amendment’s shield is set down when, as he memorably articulated, someone yells “Fire!” in a crowded theater. The jurist worried that Schenck could have disrupted conscription.    

The Espionage Act’s wartime genesis is visible in its targeting of “Whoever, for the purpose of obtaining information respecting the national defense with intent or reason to believe that the information is to be used to the injury of the United States, or to the advantage of any foreign nation.” That intent requirement is likely to be contested by Mr. Trump.      

The Espionage Act endures. In addition to Mr. Trump, the list of those charged under the Espionage Act include the analyst Daniel Ellsberg, who leaked the Pentagon Papers, and another analyst, Edward Snowden, who leaked documents relating to wiretapping by the National Security Agency. If he returns from Moscow, he faces trial under the law’s provisions.

The list of those charged under the Espionage Act reads like a Who’s Who of icons of radicalism of the last 100 years. There is the anarchist Emma Goldman, the labor leader, socialist, and presidential candidate Eugene Debs, who in 1920 ran for president after being convicted and while serving in prison. The Republican president, Warren Harding, commuted Debs’ sentence and received him at the White House.

The Rosenbergs, Julius and Ethel, went to  the chair in 1953 after being convicted under Section 2 of the Espionage Act, which prescribes life imprisonment or death to anyone convicted of transmitting or attempting to transmit to a foreign government “information relating to the national defense.” Archival material indicates they were in cahoots with the Soviets.

The founder of WikiLeaks, Julian Assange, is also on the run from Espionage Act charges. One of the most famous figures convicted under the Espionage Act, an enlisted soldier in the United States Army, Private Chelsea Manning, was convicted under its provisions. The private’s sentence was commuted by President Obama.

In an open letter, the New York Times, The Guardian, Le Monde, Der Spiegel, and El País all urged the Department of Justice to drop charges against Mr. Assange. They argued that the Espionage Act is an “old law” designed to “prosecute potential spies during World War I” and that it sets “a dangerous precedent.” 

On Friday, though, the Times editorial board ventured that “it is hard to overstate the gravity of the criminal indictment issued against Donald Trump late Thursday by a federal grand jury” for allegedly violating the Espionage Act.