Pacifists of the 20th century had a lot of blood on their hands for weakening the Western democracies in the face of rising belligerence and military might in aggressor nations like Nazi Germany and imperial Japan. In Britain during the 1930s, Labor Party members of Parliament voted repeatedly against military spending, while Hitler built up the most powerful military machine in Europe. Students at leading British universities signed pledges to refuse to fight in the event of war. Remember the Star Trek: TOS episode with Joan Collins. She played the pacifist, Edith Keeler. Gist of the episode, is that a rabid Dr. McCoy travels back into the past through an alien artifact, and changes history. Capt Kirk and Mr. Spock have to go back and repair the damage that Dr. McCoy caused to time. Edith Keeler was the pacifist who will start and effective pacifist movement that will delay the United States' entrace into World War II, thus allowing Hitler's Germany to develop the atomic bomb first and conquer the planet.
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By far "The City on the Edge of Forever" was the best Trek episode ever, and perhaps the best science fiction short story ever.
It also was one of the few Star Trek TOS episodes with a less than happy ending.