Brandenburg v. Ohio (1969)
A leader of the Ku Klux Klan was arrested after he made a speech at a Klan rally and charged under an Ohio law that made it illegal to advocate "crime, sabotage, violence, or unlawful means of terrorism..."  The Supreme Court struck down this law as overbroad.  It ruled that speech could only be prohibited if it is "directed at inciting or producing imminent lawless action" and is "likely to incite or produce such action."  Mere advocacy which does not incite imminent lawless action is protected by the First Amendment.

http://www.oyez.org/cases/1960-1969/1968/1968_492/