Final answer:
Justice Oliver Wendell Holmes concluded in the 1919 case of Schenck v. United States that speech presenting a 'clear and present danger' is not protected by the First Amendment, particularly during wartime, as manifested in Charles Schenck's anti-draft pamphlets.
Step-by-step explanation:
In the legal case of Schenck v. United States, Justice Oliver Wendell Holmes declared that the First Amendment did not prevent Congress from prohibiting speech that presented a "clear and present danger".
This pivotal 1919 case arose out of Charles Schenck's actions during World War I when he distributed leaflets urging young men to resist the draft, which was considered a violation of the Espionage Act.
Holmes, writing for a unanimous Supreme Court, stated that in certain circumstances, such as during wartime, speech that poses a significant threat to national security or the safety of the public is not protected.
This established the 'clear and present danger' test as a standard for evaluating restrictions on speech under the First Amendment.