In February 1915 fifteen hundred women delegates representing Austria, Belgium, Canada, Denmark, Germany, Britain, Hungary, Italy, Holland, Norway, Sweden and the United States met in Amsterdam to discuss the First World War.
At the meeting the women discussed ways of ending the war . Delegates also spoke about the need to introduce measures that would prevent wars in the future such as international arbitration and the state nationalization of munitions. As a result of the conference an International Women's Peace Party was formed.
Mary Sheepshanks was the leading anti-war figure in Britain. On 14th October, 1914, she wrote in Ius Suffragii: "Each nation is convinced that it is fighting in self-defence, and each in self-defence hastens to self-destruction. The military authorities declare that the defender must be the aggressor, so armies rush to invade neighbouring countries in pure defence of their own hearth and home, and, as each Government assures the world, with no ambition to aggrandise itself. Thousands of men are slaughtered or crippled... art, industry, social reform, are thrown back and destroyed; and what gain will anyone have in the end? In all this orgy of blood, what is left of the internationalism which met in congresses, socialist, feminist, pacifist, and boasted of the coming era of peace and amity. The men are fighting; what are the women doing? They are, as is the lot of women, binding up the wounds that men have made."
At a Council meeting of the National Union of Women's Suffrage Societies held in February 1915, Millicent Fawcett attacked the peace efforts of people like Mary Sheepshanks. Fawcett argued that until the German armies had been driven out of France and Belgium: "I believe it is akin to treason to talk of peace." After a stormy executive meeting in Buxton all the officers of the NUWSS (except the Treasurer) and ten members of the National Executive resigned. This included Chrystal Macmillan, Kathleen Courtney, Catherine Marshall, Eleanor Rathbone and Maude Royden, the editor of the The Common Cause.
In the autumn of 1915 women in Britain who attended the meeting in Amsterdam, including Emmeline Pethick-Lawrence and Chrystal Macmillan, formed the Women's International League of Great Britain. Other women who joined this group included Sylvia Pankhurst, Mary Sheepshanks, Charlotte Despard, Helen Crawfurd, Mary Barbour, Agnes Dollan, Ethel Snowden, Henry Harben, Ellen Wilkinson, Margery Corbett-Ashby, Selina Cooper, Helena Swanwick and Olive Schreiner.