Smith Act trials of Communist Party leaders
The Smith Act trials of Communist Party leaders were a series of trials held from 1949 to 1958 in which leaders of the Communist Party of the United States (CPUSA) were accused of violating the Smith Act, a 1940 statute that set penalties for advocating the violent overthrow of the government. The defendants argued that they advocated a peaceful transition to socialism, and that the First Amendment's guarantee of freedom of speech and of association protected their membership in a political party.
Source: Wikipedia — Smith Act trials of Communist Party leaders (CC BY-SA 4.0)