Justice Oliver Wendell Holmes defined the clear and present danger test in 1919 in Schenck v. United States, offering more latitude to Congress for restricting speech in times of war, saying that when words are "of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent....no court could regard them as protected by any constitutional right." (Photo of Holmes circa 1924 via Wikimedia Commons, public domain.)

Early in the 20th century, the Supreme Court established the clear and present danger test as the predominant standard for determining when speech is protected by the First Amendment.

The Court crafted the test — and the bad tendency test, with which it is often conflated or contrasted — in cases involving seditious libels, that is, criticisms of the government, its officials, or its policies. It would be superseded by the imminent lawless action test in the late 1960s.

 

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https://www.mtsu.edu/first-amendment/article/898/clear-and-present-danger-test