In Schenck v United States 1919, the US Supreme Court coined the term "clear and present danger" to cover "free speech" cases that could reasonably result in harm or danger.
Charles Schenck and Elizabeth Baer were members of the Executive Committee of the Socialist Party in Philadelphia during World War I. They printed leaflets with the messages, "Do not submit to intimidation", "Assert your rights", "If you do not assert and support your rights, you are helping to deny or disparage rights which it is the solemn duty of all citizens and residents of the United States to retain." They were urging men to refuse the draft.
Jury trials convicted Schenck and Baer of violating Section 3 of the Espionage Act of 1917 and they appealed to the US Supreme Court. They argued that the Espionage Act of 1917 went against the First Amendment if people are forbidden to exercise free speech in speech and text. Justice Oliver Wendell Holmes, Jr wrote:
"..when a nation is at war, many things that might be said in time of peace are such a hindrance to its effort that their utterance will not be endured so long as men fight, and that no Court could regard them as protected by any constitutional right."
"The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic. [...] The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent.