Clear and present danger is difficult to define concisely. Your Dictionary calls it “the principle that the government, notwithstanding the First Amendment to the United States Constitution, may restrict, prohibit, or punish speech or the printing and distribution of words if it is necessary to prevent a clear and present danger of an event that the government has a right to prevent.” (The definition includes the term, but that fact can be overlooked by anyone who knows what the words mean separately). Ultimately, what this means is that a clear and present danger to public interests is the only threat that justifies an abridgment of free speech.
As inconvenient as anti-war protesting may be to the administration that favors military action, it does not constitute a clear and present danger to the republic.
The origin of this limit on First Amendment freedom is found in Schenck v. United States, in which seditious speech was quelled despite the argument of First Amendment protection. In 1919, Charles Schenck of the American Socialist Party admitted to having distributed some 15,000 anti-draft leaflets to new recruits, encouraging them to protest what he called “involuntary servitude.” Although Schenck insisted that it was his right to make even inflammatory political statements, the court found him in violation of the Espionage Act of 1917. Justice Oliver Wendell Holmes explained the ruling: “The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic…The question in every case is whether the words used are used in such circumstances and 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.” (While political speech is not prohibited per se, or even without great difficulty, a military uprising during World War I was taken to qualify as a clear and present danger to national security.)
Other Important Information
Since Schenck v. United States, clear and present danger has remained difficult to prove except in cases like those mentioned by Justice Holmes (shouting “fire” in a theatre and distributing seditious leaflets). Mere instigative speech is not sufficient. For example, shortly after World War II, Arthur Terminiello delivered a speech denouncing the Jews and praising Hitler’s recent actions. His words predictably started a riot, but Justice Douglas ruled that they nevertheless did not threaten “serious substantive evil.” A clear and present danger must be a danger, and not only an inconvenience.