In 1919, Jane Addams founded with others the Women's International League for Peace and Freedom (WILF).
During World War I, she chaired the International Congress of Women held in the Hague in the Netherlands in 1915; in attendance at the congress was Edinburgh University's Chrystal Macmillan. The Congress sent a delegation to heads of neutral states calling for an end to the war. Addams went on to petition the US President, Woodrow Wilson, to ask that he mediate peace between the warring countries. Her call went unheeded. When the USA entered the war instead, Jane Addams campaigned against this.
In 1919, a second International Congress of Women was held, this time in Zurich; Chrystal Macmillan was again a delegate. The Congress condemned the 'peace terms' being planned for Germany in the Treaty of Versailles, and Chrystal carried the Congress's condemnation to the ongoing Paris Peace Treaty - to no effect. The organisation established by the 1919 Congress was named the Women's International League for Peace and Freedom.
Jane Addams was subsequently awarded the Nobel Peace Prize in 1931; she was the second woman to receive this award.
Source: Nobelprize.org; http://wilpf.org