principal U.S. legislation prohibiting espionage for a foreign country and providing heavy penalties for such activity. As amended in 1940 and 1970, it is still in force.

The 1917 law provided steep fines and imprisonment for collecting and transmitting to a foreign power information related to U.S. national defense and for interfering with the recruitment or loyalty of the armed forces. Use of the U.S. mail for material urging treason or resistance to U.S. laws was prohibited; sabotage, especially of trading ships, was subjected to severe penalties; the movement of neutral ships in U.S. waters was regulated (an attempt to stop such vessels from shipping arms or supplies to an enemy country); and the fraudulent use of passports as well as the unauthorized representation of a foreign government were prohibited. An important amendment to the law, usually called the Sedition Act, was passed in 1918 but repealed in 1921; it forbade spoken or printed attacks on the U.S. government, Constitution, or flag.

During the outbreak of public hysteria early in World War I, the 1917 and 1918 laws permitted about 1500 trials and prison sentences; freedom of the press was curtailed. In 1919 this led U.S. Supreme Court justices Oliver Wendell Holmes, Jr., and Louis Brandeis to state the principle, later much cited, that unpatriotic speech and publications were illegal only if they constituted a “clear and present danger” to national security.

The 1940 revision of the Espionage Act increased its penalties. The application of the law to mere propaganda was limited by a Supreme Court decision in 1944. During World War II about 160 people were convicted under the Espionage Act. Also under this act, the American Communists Julius Rosenberg (b. 1918) and Ethel Rosenberg (b. 1915) were convicted of spying and executed in 1953.