In 1919, the U.S. Supreme Court decided that context is everything when it comes to protected speech. Specifically, you cannot say anything that might incite others to some type of lawless action, or to an action that would harm others, in the very near future ("clear and present danger"). The famous example used to explain this speech prohibition came from Supreme Court Justice Oliver Wendell Holmes, who likened it to falsely shouting, "Fire!" in a crowded theater. You can shout "Fire!" in your home or backyard, but not in an enclosed, crowded place where such language could cause a panic and possibly injury and death. Similarly, you can't, say, egg on a crowd of angry, young men to attack another person [sources: McBride, Volokh].
The case that spurred this decision was United States v. Schenck. Charles Schenck was a socialist who tried to pass out anti-draft flyers to newly drafted American servicemen during World War I. His flyers said the draft was the same as slavery, a practice outlawed in the Constitution's 13th Amendment, and told new draftees to try to repeal the draft. Police charged Schenck with violating the country's new Espionage Act, and a jury later convicted him.
Schenck appealed on the grounds that the Espionage Act was illegal because it violated the First Amendment's free-speech provisions. The Court ruled against Schenck due to context: It's fine to pass out such flyers during peacetime, but not wartime, when they could incite national insubordination. This ruling stood until 1969 when the Court said that the "imminent lawless action" test could only allow the government to limit free speech when it incited unlawful action to take place sooner than police could arrive to prevent it [source: McBride].