Final answer:
The 'clear and present danger' test, especially as applied in Brandenburg v. Ohio, establishes when speech can be restricted under the First Amendment, barring incitement to immediate unlawful action and speech posing clear and present danger.
Step-by-step explanation:
The legal standard that states the First Amendment does not protect speech with a tendency to incite unlawful activity revolves around the 'clear and present danger' test. This was most notably applied in the case of Brandenburg v. Ohio, which determined that speech advocating the use of force or law violation is protected unless it incites immediate unlawful action. Incitement to imminent lawless action and speech presenting a clear and present danger are key thresholds that restrict First Amendment protection.
In cases like Schenck v. United States and Chaplinsky v. New Hampshire, the Supreme Court has ruled on the extent to which speech can be legally restricted despite the First Amendment's protections. For example, 'fighting words' and speech that is directly linked to a conspiracy to overthrow the government are not protected. However, abstract discussions or criticisms of the government without a direct incitement of violence remain protected under the First Amendment.