The Dennis et al. v. United States case in 1950 revolved around the issue of whether advocating for the violent overthrow of the U.S government was protected under free speech rights as per First Amendment or not. The defendants, leaders of the Communist Party USA, were charged with violating the Smith Act - a federal statute that made it unlawful to knowingly conspire to teach and advocate for overthrowing or destroying any government in America by force or violence. They argued that they had only been discussing Marxist-Leninist principles rather than actively plotting an insurrection against American governance. However, their conviction was upheld by Supreme Court on grounds that such advocacy created a "clear and present danger" to national security which outweighed First Amendment protections; thus marking one of few instances where limitations on freedom of speech were deemed constitutional.

In the dissenting opinion for Dennis et al. v. United States, Justice Hugo Black and Justice William O. Douglas argued that the Smith Act, under which the defendants were prosecuted, was unconstitutional as it violated First Amendment rights to free speech and assembly. They contended that advocating for a doctrine of overthrowing or destroying government by force or violence is not equivalent to an actual attempt at incitement; thus, mere advocacy should be protected under freedom of speech unless there's clear evidence of imminent danger posed by such advocacy. The justices also criticized the majority's reliance on balancing national security against individual liberties in this case because they believed it could potentially lead to suppression of political dissent in times when national security threats are perceived but not necessarily real or immediate.