Which case involved the limitation of speech during wartime based on "clear and present danger"?

The case that involved the limitation of speech during wartime based on the principle of "clear and present danger" is Schenck v. United States. This landmark Supreme Court decision, decided in 1919, addressed the conflict between free speech rights and national security during World War I. Charles Schenck was convicted for distributing pamphlets that encouraged resistance to the military draft, which the government interpreted as a threat to the country’s war efforts.

The Court, in a unanimous opinion authored by Justice Oliver Wendell Holmes Jr., established that during times of war, speech that poses a "clear and present danger" to national security can be restricted. This means that if an individual's speech is likely to incite actions that would harm the nation—such as encouraging draft evasion—then the government has the authority to limit that speech to protect the nation. This case set a significant precedent for future cases involving the First Amendment and the limits of free expression, particularly in contexts where national safety is at stake.