The Gist: The Espionage Act of 1917 made it a criminal act to interfere with military operations or support U.S. enemies during wartime.During World War I, a couple of activists against the war were convicted for handing out fliers that discouraged people from complying with the draft. The Supreme Court held that the activists criticism of the war was not protected by the First Amendment because their actions constituted a "clear and present" danger to military recruitment. 

The Details:

The Espionage Act only applied to to actions that successfully obstructed military operations, and there was no proof in this case to establish the leaflets handed out by these activists were actually successful in discouraging people from complying with the draft. Nevertheless, the court reasoned that the Espionage Act would still be upheld and the activists convicted because their actions, even though it was only through speech, were dangerous enough that they could lead to success. The Court also recognized that in times of war, protection of the nation took priority over protection of  dangerous speech. It was in this case that the "clear and present danger" standard was established:

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.

The "clear and present danger" standard in limiting freedom of speech would remain the test for balancing the government's interests against the interests of speech for 50 years. Of course, in this decision and in decisions to follow, the main criticism was and would become ambiguity in what constituted a "clear" and/or "present" "danger."  Other controversial cases during this time, where the court upheld the Act against  those speaking out against the war, were Debs v. United States, 249 U.S. 211 (1919) and Frohwerk v. United States, 249 U.S. 204 (1919) and Abrams v. United States, 250 U.S.  616 (1919).