TL;DR: during WW1, Socialist Party members protested against the war, and particularly against the draft. As part of those protests, they handed out leaflets that encouraged those eligible to resist the draft. The government cracked down on it, and the Supreme Court ruled in favor of the government, likening the action to "shouting fire in a theatre and causing a panic".
Thankfully, the present standard is "imminent lawless action" instead, which is a far higher bar for the government to meet if it wants to regulate speech.
https://en.wikipedia.org/wiki/Schenck_v._United_States
TL;DR: during WW1, Socialist Party members protested against the war, and particularly against the draft. As part of those protests, they handed out leaflets that encouraged those eligible to resist the draft. The government cracked down on it, and the Supreme Court ruled in favor of the government, likening the action to "shouting fire in a theatre and causing a panic".
Thankfully, the present standard is "imminent lawless action" instead, which is a far higher bar for the government to meet if it wants to regulate speech.
https://en.wikipedia.org/wiki/Brandenburg_v._Ohio