 Following Congressional investigation of left @-@ wing and right @-@ wing extremist political groups in the mid @-@ 1930s , support grew for a statutory prohibition of their activities . The alliance of Nazi Germany and the Soviet Union in the August 1939 Molotov – Ribbentrop Pact and their invasion of Poland in September gave the movement added impetus . In 1940 the Congress passed the Alien Registration Act of 1940 ( known as the Smith Act ) which required all non @-@ citizen adult residents to register with the government , and made it a crime " to knowingly or willfully advocate ... the duty , necessity , desirability , ... of overthrowing or destroying any government in the United States by force or violence ... with the intent to cause the overthrow or destruction of any government in the United States .... " Five million non @-@ citizens were fingerprinted and registered following passage of the Act . The first persons convicted under the Smith Act were members of the Socialist Workers Party ( SWP ) in Minneapolis in 1941 . Leaders of the CPUSA , bitter rivals of the Trotskyist SWP , supported the Smith Act prosecution of the SWP – a decision they would later regret . In 1943 , the government used the Smith Act to prosecute American Nazis ; that case ended in a mistrial when the judge died of a heart attack . Anxious to avoid alienating the Soviet Union , then an ally , the government did not prosecute any communists under the law during World War II . 
