Final answer:
Schenck v. United States established the 'clear and present danger' test for limiting free speech under the First Amendment, ruling that Charles Schenck's anti-draft distributions during WWI were not protected as they posed a threat to national security.
Step-by-step explanation:
The case of Schenck v. United States is significant as it set a precedent regarding the extent to which the First Amendment protects free speech. In 1919, the U.S. Supreme Court, led by Justice Oliver Wendell Holmes, ruled unanimously that Charles Schenck's actions in distributing anti-draft leaflets during World War I were not protected under the free speech clause of the First Amendment. The ruling established the 'clear and present danger' test, indicating that free speech is limited when it poses a significant threat to national security or public order.
Holmes articulated that in times of war, expressions that would be tolerable in peacetime could be punished if they create a clear and present danger of bringing about the substantive evils that Congress is entitled to prevent. Ultimately, this case affirmed that the right to free speech is not absolute and can be legally restricted under certain grave circumstances.