The idea of falsely shouting "fire" in a crowded theater arose from the Supreme Court’s 1919 decision in the case Schenck v. United States. The Court ruled unanimously that the First Amendment, though it protects freedom of expression, does not protect dangerous speech. In the decision, Oliver Wendell Holmes wrote that no free speech safeguard would cover someone "falsely shouting fire in a theater and causing a panic."The case in question did not involve fires, theaters or general panic. It instead concerned a man’s conviction for protesting the First World War’s military draft. The man, Charles Schenck, had printed 15,000 fliers that encouraged readers to resist conscription. The Espionage Act of 1917 and the Sedition Act of 1918 criminalized such an offense, said prosecutors.
Schenck argued that the Constitution allowed his expression, but the Court disagreed. According to their ruling, Schnenck’s fliers created a clear and present danger — a clear and present danger to the government’s recruiting efforts. He hadn’t endangered life, as falsely shouting "fire" in a crowded theater would have, but he may as well have.
This "clear and present danger" standard stood for half a century. Further rulings even expanded it, criminalizing additional speech. But the Supreme Court then heard a case involving a new example of questionable speech, one that modern sensibilities might find more controversial than war protests.
Charles Brandenburg, a Ku Klux Klan leader, had spoken to group members at a televised Ohio rally. He’d used inflammatory language and racial slurs. He’d called for "revengeance," which Ohio prosecutors interpreted as a call to violence. This meant, said the prosecutors, that Charles Brandenburg had broken the law.
A statute, which the state had enacted the same year as the Schenck decision, criminalized the advocacy of crime or violence. The victims of any possible crime this speech incited would face even clearer danger than patrons fleeing a crowded theater.
Yet Brandenburg claimed the First Amendment protected his speech. His appeal reached the Supreme Court, and the Court agreed with him, in contrast with the earlier Schenck decision. Advocacy, even when it encourages law-breaking, helps the marketplace of ideas, ruled the Court. Had Brandenburg instructed followers to commit a specific crime, he’d have committed a number of offenses himself. But the First Amendment protects speech that merely advocates general, indefinite illegal action.
With that ruling, the Court overturned the Schenck decision that had introduced "shouting fire in a crowded theater." No longer was "clear and present danger" a sufficient standard for criminalizing speech. To break the law, speech now had to incite "imminent lawless action."
So if a court can prove that you incite imminent lawlessness by falsely shouting "fire" in a crowded theater, it can convict you. If you incite an unlawful riot, your speech is "brigaded" with illegal action, and you will have broken the law. But merely falsely shouting "fire" does not break the law, even if it risks others’ safety.And, of course, no court will fault you for warning of a fire that actually exists.