If you're interested in British opposition to WWI, Nobel prize winner in literature and philosopher Bertrand Russell, who later also opposed the Vietnam war, writes about his activism in his memoires. They were widely considered unpatriotic and there's a poignant anectode of how one of their meetings was attacked by an enraged crowd and the police only intervened to save him after someone explained to them he was a nobleman.

The literature on German opposition to the war is also interesting, especially in the context of the left, with arguments from different sides that the left were variously traitors for abandoning their ideals in supporting the war and that they were traitors to their country for not supporting the war (the Dolkstusslegende, interesting in itself in the context of the Nazi rise to power).

The “fire in a crowded theater” language that many people cite (incorrectly – it is not the modern legal standard) when discussing limits to the protections afforded by the First Amendment arose from prosecution of [Charles Schenck] (http://www.law.cornell.edu/supremecourt/text/249/47) for his opposition to the draft in WWI.
Schenck was the general secretary of the Socialist Party of the United States and was charged under the Espionage Act for distributing a pamphlet that urged people to oppose the draft. Schenck v. United States was one of a trio of opinions by Justice Holmes in which the Supreme Court upheld convictions for opposition to the draft. The other cases are Debs v. United States and Frohwerk v. United States.