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World War I

Although President Woodrow Wilson was re-elected in 1916 under the slogan "He kept us out of the war", at the start of his second term, he announced that Germany's continued deployment of unrestricted submarine warfare was sufficient cause for the US to enter the Great War. Shortly afterward, Congress passed the Selective Service Act of 1917, which required all males aged 21–30 to register for military conscription. Goldman saw the decision as an exercise in militarist aggression, driven by capitalism. She declared in Mother Earth her intent to resist conscription, and to oppose US involvement in the war.

To this end, she and Berkman organized the No Conscription League of New York, which proclaimed: "We oppose conscription because we are internationalists, antimilitarists, and opposed to all wars waged by capitalistic governments." The group became a vanguard for anti-draft activism, and chapters began to appear in other cities. When police began raiding the group's public events to find young men who had not registered for the draft, Goldman and others focused their efforts on distributing pamphlets and other writings. In the midst of the nation's patriotic fervor, many elements of the political left refused to support the League's efforts. The Women's Peace Party, for example, ceased its opposition to the war once the US entered it. The Socialist Party of America took an official stance against US involvement but supported Wilson in most of his activities.

On June 15, 1917, Goldman and Berkman were arrested during a raid of their offices, in which authorities seized "a wagon load of anarchist records and propaganda". The New York Times reported that Goldman asked to change into a more appropriate outfit, and emerged in a gown of "royal purple". The pair were charged with conspiracy to "induce persons not to register" under the newly enacted Espionage Act, and were held on US$25,000 bail each. Defending herself and Berkman during their trial, Goldman invoked the First Amendment, asking how the government could claim to fight for democracy abroad while suppressing free speech at home:

We say that if America has entered the war to make the world safe for democracy, she must first make democracy safe in America. How else is the world to take America seriously, when democracy at home is daily being outraged, free speech suppressed, peaceable assemblies broken up by overbearing and brutal gangsters in uniform; when free press is curtailed and every independent opinion gagged? Verily, poor as we are in democracy, how can we give of it to the world?

The jury found Goldman and Berkman guilty. Judge Julius Marshuetz Mayer imposed the maximum sentence: two years' imprisonment, a $10,000 fine each, and the possibility of deportation after their release from prison. As she was transported to Missouri State Penitentiary, Goldman wrote to a friend: "Two years imprisonment for having made an uncompromising stand for one's ideal. Why that is a small price."

In prison, she was assigned to work as a seamstress, under the eye of a "miserable gutter-snipe of a 21-year-old boy paid to get results". She met the socialist Kate Richards O'Hare, who had also been imprisoned under the Espionage Act. Although they differed on political strategy—O'Hare believed in voting to achieve state power—the two women came together to agitate for better conditions among prisoners. Goldman also met and became friends with Gabriella Segata Antolini, an anarchist and follower of Luigi Galleani. Antolini had been arrested transporting a satchel filled with dynamite on a Chicago-bound train. She had refused to cooperate with authorities and was sent to prison for 14 months. Working together to make life better for the other inmates, the three women became known as "The Trinity". Goldman was released on September 27, 1919.