Americans may take free speech for granted, but they couldn’t do so a century ago. Courts convicted newspapermen, pamphleteers and politicians for nothing more—and sometimes less—than trying to sway the public against U.S. involvement in World War I. On Nov. 10, 1919, the Supreme Court affirmed the conviction of antiwar protesters under a law that made it a crime to “hinder” the war effort. But a dissent in Abrams v. U.S. laid the foundation for today’s robust protection of controversial speech.

The idea that speech could pose a “clear and present danger” to the government, and thus lacked First Amendment protection, came from a quartet of 1919 cases, three of which were unanimous. In March, in Schenck v. U.S., the court, led by archprogressive Justice Oliver Wendell Holmes Jr., upheld the convictions of pamphleteers who encouraged draft-dodging. A week later, Frohwerk v. U.S. upheld the conviction of a newspaperman who criticized U.S. involvement in foreign wars, while Debs v. U.S. affirmed the conviction of Socialist Party leader Eugene Debs for denouncing the war in a speech. (Debs went on to receive 3.4% of the 1920 presidential vote from prison.)

In October the court in Abrams upheld another antiwar protest conviction—but this time not unanimously. Like Charles Schenck, Jacob Abrams was a socialist who had distributed antiwar pamphlets. His group criticized U.S. military support for the anti-Communist White movement in the Russian Civil War. As socialists of the time often did, the pamphleteers urged a general strike in New York, on grounds that workers were making weapons to use against their Bolshevik comrades.

But those same weapons had an alternative—and likelier—use in the allied war effort against the Germans. The defendants were convicted under the Sedition Act of 1918, which made it a crime to “urge, incite, or advocate any curtailment of production … with intent by such curtailment to cripple or hinder … the prosecution of the war.”

The court affirmed the convictions over the surprising dissent of Schenck’s author, Holmes, joined by Justice Louis Brandeis. It’s unclear why Holmes put aside his majoritarian impulses to support a constitutional restriction on legislative action. Some scholars have posited that Holmes acted not out of a commitment to the Constitution but because of the disparate impact of speech restrictions on progressive icons. Even so, the dissent is steeped in libertarian ideas. It draws implicitly on John Stuart Mill and asserts that “the best test of truth is the power of the thought to get itself accepted in the competition of the market.”

Abrams and the other cases were effectively overturned in Brandenburg v. Ohio (1969), which held that speech can be prosecuted only if intended to foment “imminent lawless action.” Since then, the court has vigorously defended speech that offends or provokes, ranging from the publication of the Pentagon Papers, to violent rap lyrics, from pornographic humor to burning the American flag.

These strong protections give Americans confidence in our right to express and hear unpopular opinions. The experience of a century ago should warn us against being complacent about it.