In 1914, shortly after the start of World War I, Jacobs promoted holding the International Women's Congress in The Hague, given the country's neutrality. Intended as a forum for women from throughout the world to meet and discuss opposition to war, the meeting was chaired by the pacifist Jane Addams from Chicago. Coordinated by Jacobs, Mia Boissevain, and Rosa Manus, the conference, which opened on 28 April 1915, was attended by 1,136 participants from both neutral and non-belligerent nations, and resulted in the establishment of an organization which would become the Women's International League for Peace and Freedom (WILPF). Jacobs became the vice-president of both the international organization and the Dutch chapter of WILPF. After the conference closed on 3 May 1915, Addams and Jacobs, along with Chrystal Macmillan, Rosika Schwimmer and Mien van Wulfften Palthe-Broese van Groenou and others, formed two delegations of women who were dispatched to meet European heads of state over the next several months. The women secured agreement from reluctant Foreign Ministers, who overall felt that such a body would be ineffective. Nevertheless, they agreed to participate, or not impede creation of a neutral mediating body, if other nations agreed and if US President Woodrow Wilson would initiate a body. In the midst of the war, Wilson refused.