Clear and present danger

"Words which, ordinarily and in many places, would be within the freedom of speech protected by the First Amendment may become subject to prohibition when of such a nature and used in such circumstances a to create a clear and present danger that they will bring about the substantive evils which Congress has a right to prevent. The character of every act depends upon the circumstances in which it is done."

See: text and commentary (http://faculty-web.at.nwu.edu/commstud/freespeech/cont/cases/schenck/schenckframe)

Clear and Present Danger is also a book by Tom Clancy, which was made into a movie starring Harrison Ford.