When the United States finally decided to enter World War I in 1917, there was opposition at home by those who wanted America to remain neutral in the European conflict and groups who actively opposed the draft, the first of its kind in the country. The most vocal dissent came from pacifists, anarchists and socialists, many of whom were Irish, German and Russian immigrants and whose loyalty to America was openly questioned.

Fearing that anti-war speeches and street pamphlets would undermine the war effort, President Woodrow Wilson and Congress passed two laws, the Espionage Act of 1917 and the Sedition Act of 1918, that criminalized any “disloyal, profane, scurrilous, or abusive language” about the U.S. government or military, or any speech intended to “incite insubordination, disloyalty, mutiny, or refusal of duty.” (These were different and separate from the Alien and Sedition Acts passed in 1798 that were mostly repealed or expired by 1802.)

The two broadly worded laws of 1917 and 1918 ultimately came to be viewed as some of the most egregious violations of the Constitution’s free speech protections. They were written in an environment of wartime panic and resulted in the arrest and prosecution of more than 2,000 Americans, some of whom were sentenced to 20 years in prison for sedition.

A handful of those convictions were appealed to the Supreme Court, which upheld the Espionage and Sedition Acts as constitutional limits on free speech in a time of war. One famous decision penned by Justice Oliver Wendell Holmes introduced the “clear and present danger” test, which he compared to shouting “fire!” in a crowded theater.

The Wilson administration knew that many Americans were conflicted about the U.S. entry into World War I, so it launched a sweeping propaganda campaign to instill hatred of both the German enemy abroad and disloyalty at home. Wilson publicly stated that disloyalty to the war effort “must be crushed out” and that disloyal individuals had “sacrificed their right to civil liberties” like free speech and expression.

The Espionage Act of 1917 was passed just two months after America entered World War I and was primarily intended by Congress to combat actual espionage on behalf of America’s enemies, like publishing secret U.S. military plans. But federal prosecutors and judges, following Wilson’s lead, fixated on Section 3 of the Espionage Act, which targeted individuals who “willfully cause or attempt to cause insubordination, disloyalty, mutiny, [or] refusal of duty” in the military.

The law gave the U.S. Postmaster General the authority to block the mailing of any letter, pamphlet or book seen as opposing or questioning America’s military involvement in World War I. That led to investigations and prosecutions of everyone from unknown street pamphleteers to Eugene Debs, America’s most prominent socialist and labor organizer.

As the war rolled on and more American soldiers died, Congress doubled down on disloyal speech and passed the Sedition Act of 1918, which amended and expanded on the Espionage Act to target any speech that could be interpreted as criticizing the war effort, the draft, the U.S. government or the flag.

“The whole reason behind the Espionage Act and the Sedition Act was the fact that the government understood that words matter, words had influence,” says Lon Strauss, an assistant professor of military history and war studies at the Marine Corps Command and Staff College.

“They were definitely concerned that folks who were against American participation in the war could influence drafted men. They didn’t want the fighting will of the American soldier to be sapped.”

The Supreme Court, which the Founding Fathers intended to be a check on unconstitutional pieces of legislation, proved that it also wasn’t immune to wartime fears of insurrection at home.

“The Supreme Court accepted broad interpretations of both the Espionage Act and the Sedition Act, and in a series of cases upheld convictions as consistent with the First Amendment,” says Geoffrey Stone, a professor at the University of Chicago Law School and author of Perilous Times: Free Speech in Wartime.

One of the Court’s landmark decisions was Schenck v. United States, in which socialist Charles Schenck was charged with conspiracy to violate the Espionage Act by distributing leaflets urging Americans to disobey the draft. The Court voted unanimously to uphold the conviction, citing necessary limits on free speech during times of war.

“The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent,” wrote Justice Holmes, introducing a new judicial test for whether speech crossed the line from disloyal to dangerous.

To illustrate what kind of speech met the “clear and present danger” test, Holmes gave a now-famous hypothetical example. "The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic,” he wrote.

Holmes and his fellow justices upheld convictions in two more conspiracy cases, including Debs v. United States, in which the outspoken socialist and presidential candidate was imprisoned for simply pledging support for three men who had been jailed for violating the Espionage and Sedition Acts.

Shortly after his arrest, Debs wrote a friend, “I am expecting nothing but conviction under a law flagrantly unconstitutional and which was framed especially for the suppression of free speech.”

Then something interesting happened. Holmes and another justice, Louis Brandeis, appear to have had a change of heart.

In Abrams v. United States, argued before the Supreme Court a year after the end of World War I, the justices were split. At issue was the conviction of two Russian immigrants who threw leaflets from an apartment window in 1918 denouncing U.S. interference in the Bolshevik Revolution. Seven justices claimed that the action met the “clear and present danger” test, but not Holmes and Brandeis.

Writing for the minority, Holmes presented a new judicial philosophy for regulating speech, in which ideas—good or bad, benign or dangerous—are free to compete in a marketplace of ideas.

"The ultimate good desired is better reached by free trade in ideas,” wrote Holmes, “that the best test of truth is the power of the thought to get itself accepted in the competition of the market, and that truth is the only ground upon which their wishes safely can be carried out."

It would take decades before the full Court embraced Holmes’ “marketplace of ideas_._” In 1969, the justices ruled in Brandenburg v. Ohio that even speech by Ku Klux Klan members advocating violence was protected.

The Sedition Act, however, didn’t last nearly as long. Congress repealed the law in 1920 along with a host of wartime restrictions, and most prisoners convicted under the Sedition Act, including Debs, were released.

“Today, it’s unequivocal,” says Stone. “You cannot punish someone for speech that may cause others to engage in unlawful conduct unless you can show at the very least that it created a clear and present danger of grave harm, and the Supreme Court has not upheld a single conviction under that standard in 50 years.”

The Espionage Act never fully went away. A portion of it survives in the federal criminal code as “seditious conspiracy,” defined as two or more people conspiring to “overthrow, put down, or to destroy by force” the U.S. government, to illegally seize property or prevent the execution of any law. In fact, in September 2020, Attorney General William P. Barr was reported to have asked prosecutors to consider charging rioters and others who had committed violent crimes at protests with sedition.