Final answer:
Freedom of speech limits propaganda by disallowing speech that falsely incites panic, ensuring public safety over unrestricted expression.
Step-by-step explanation:
The limit freedom of speech imposes on propaganda is that it cannot falsely incite panic. This is well-articulated by the legal standard set in Schenck v. United States, where Justice Oliver Wendell Holmes wrote that 'the most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic.' Therefore, while freedom of speech is a fundamental right, it does not protect speech that presents a clear and present danger or falsehoods that can cause immediate harm, like inciting violence or panic.