The Dennis v. United States case in 1949 revolved around the conviction of eleven Communist Party leaders under the Smith Act, a statute that made it unlawful to advocate or organize the destruction or overthrow of any government in the U.S. by force or violence. The defendants appealed their convictions arguing that they were protected by their First Amendment rights and that there was no clear and present danger posed by them to justify such restrictions on speech. However, the Supreme Court upheld their convictions with a 6-2 majority vote stating that while abstract teaching of Marxist doctrine is not punishable, plotting to take action towards its principles is illegal under federal law. The court ruled that when national security is at stake, even probable future harm can justify limitations on freedom of speech.

In the dissenting opinion for Dennis v. United States, Justice Hugo Black and Justice William O. Douglas argued that the Smith Act violated the First Amendment's protection of free speech. They contended that advocating for a doctrine was not equivalent to inciting immediate illegal action, which is where they believed the line should be drawn in terms of restricting speech under national security concerns. The justices also expressed concern about how broadly "conspiracy" had been defined in this case, suggesting it could potentially criminalize any discussion or advocacy of unpopular ideas if those ideas were deemed threatening by those in power at any given time. Furthermore, they pointed out inconsistencies with past cases involving freedom of speech during wartime and peacetime conditions; arguing that even during times of war when national security threats are heightened, restrictions on free speech have been limited only to instances where there is a clear and present danger - something they did not see as applicable here.