On the outbreak of the First World War a group of women pacifists in the United States began talking about the need to form an organization to help bring it to an end. On the 10th January, 1915, over 3,000 women attended a meeting in the ballroom of the New Willard Hotel in Washington and formed the Woman's Peace Party. Jane Addams was elected chairman and other women involved in the organization included Mary McDowell, Florence Kelley, Alice Hamilton, Anna Howard Shaw, Belle La Follette, Fanny Garrison Villard, Mary Heaton Vorse, Emily Balch, Jeanette Rankin, Lillian Wald, Edith Abbott, Grace Abbott, Crystal Eastman, Carrie Chapman Catt, Emily Bach, and Sophonisba Breckinridge.
In April 1915, Aletta Jacobs, a suffragist in Holland, invited members of the Woman's Peace Party to an International Congress of Women in the Hague. Jane Addams was asked to chair the meeting and Mary Heaton Vorse, Alice Hamilton, Grace Abbott, Julia Lathrop, Leonora O'Reilly, Sophonisba Breckinridge and Emily Bach went as delegates from the United States. Others who went to the Hague included Emmeline Pethick-Lawrence, Emily Hobhouse, (England); Chrystal Macmillan (Scotland) and Rosika Schwimmer (Hungary). Afterwards, Jacobs, Addams, Macmillan, Schwimmer and Balch went to London, Berlin, Vienna, Budapest, Rome and Paris to speak with members of the various governments in Europe. During this time they met Edward Grey (13th May), Herbert Asquith (14th May), Gottlieb von Jagow (21st May), Theobold von Bethmann-Hollweg (22nd May), Karl von Sturgkh (26th May), Théophile Delcassé (12th June) and Rene Viviani (14th June).
Alice Hamilton visited Germany in 1915 to discover what the people thought of the war. She later recalled: "One must always remember that most Germans read nothing and hear nothing from the outside. I talked with an old friend, the wife of a professor under whom I worked years ago when I was studying bacteriology in Germany. She and her husband are people with cosmopolitan connections, they read three languages beside their own, and have always been as far removed as possible from narrow provincialism, but since last July they have known nothing except what their Government has decided that they shall know. I did not argue with my friend, but, of course, we talked much together and after she had been with us for three days she told me that she had never known before that there were people in England who did not wish to crush Germany, who wished for a just settlement, and even some who were opposed to the war."
The women were attacked in the press by Theodore Roosevelt who described them as "hysterical pacifists" and called their proposals "both silly and base". Jane Addams was selected for particular criticism. One man wrote in the Rochester Herald, "In the true sense of the word, she is apparently without education. She knows no more of the discipline and methods of modern warfare than she does of its meaning. If the woman conceded by her sisters to be the ablest of her sex, is so readily duped, so little informed, men wonder what degree of intelligence is to be secured by adding the female vote to the electorate."
By 1917 the Woman's Peace Party had 40,000 members. However, after the United States entered the war the party fragmented. The Espionage Act, passed by Congress in 1917, prescribed a $10,000 fine and 20 years' imprisonment for interfering with the recruiting of troops or the disclosure of information dealing with national defence. Additional penalties were included for the refusal to perform military duty.
Criticised as unconstitutional, the act resulted in the imprisonment of many of the anti-war movement. This included Rose Pastor Stokes who was sentenced to ten years in prison for saying, in a letter to the Kansas City Star, that "no government which is for the profiteers can also be for the people, and I am for the people while the government is for the profiteers."