Answer:
Clear and present danger refers to the legal test established in the US Supreme Court case Schenck v. United States, (1919), that attempted to determine appropriate limitations on First Amendment protection of free speech.
Charles Schenck, general secretary of the American Socialist Party was arrested and convicted for sending 15,000 anti-draft circulars through the mail to men scheduled to enter the military service. The circular called the draft law a violation of the 13th Amendment's prohibition of slavery. It went on to urge draftees not to "submit to intimidation," but to "petition for repeal" of the draft law.
In this case, the danger was determined to be a risk to the United States' recruitment and conscription efforts during WW I, and in violation of the new 1917 Espionage Act; however, the concept was used as a test in many cases through the years. The clear and present danger doctrine has been superseded by less restrictive First Amendment limitations. The current test was established in Bradenburg v. Ohio, (1969), and requires the exercise of free speech to be such that it would probably incite "imminent lawless action" (an immediate or nearly immediate act of violence or other unlawful activity).
Case Citation:
Schenck v. United States, 249 US 47 (1919)