Schenck v. United States in 1919, which held that the defendant’s speech in opposition to the draft during World War I was not protected free speech under the First Amendment of the United States Constitution. The case was later partially overturned by Brandenburg v. Ohio in 1969, which limited the scope of banned speech to that which would be directed to and likely to incite imminent lawless action (e.g. a riot).
The Court held that the government cannot punish inflammatory speech unless that speech is “directed to inciting or producing imminent lawless action and is likely to incite or produce such action”.
Semantics. You’re allowed to yell fire if there is a fire, for example.
https://en.wikipedia.org/wiki/Shouting_fire_in_a_crowded_theater
https://en.wikipedia.org/wiki/Brandenburg_v._Ohio