Slippery Slope Arguments in History: The Supreme Court in the Flag Salute Case

From Justice Jackson's majority opinion in W. Va. State Bd. of Educ. v. Barnette (1943):

As first and moderate methods to attain unity have failed, those bent on its accomplishment must resort to an ever-increasing severity…. Those who begin coercive elimination of dissent soon find themselves exterminating dissenters…. [T]he First Amendment to our Constitution was designed to avoid these ends by avoiding these beginnings.