The Supreme Court decision in Schenck v. the United States most affected

Charles Schenck and Elizabeth Baer were convicted under the 1917 Espionage Act for mailing leaflets encouraging men to resist the military draft. They appealed to the Supreme Court on the grounds that the conviction violated their free speech rights.
The Supreme Court upheld their convictions, ruling that speech that creates a “clear and present danger” (by encouraging violence or insurrection, or endangering national security) is not protected by the First Amendment.

Armed with that knowledge it affected everyone as they could walk into an air port, say they have a you know what and not get in trouble, now they would get in trouble with a whole list of charges. this also heavily affected the way criminals behaved as they have to be very careful what they say to certain people as that could pick them up an additional charge.