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World War I

Peace movements became active in the Western world after 1900, often focusing on treaties that would settle disputes through arbitration, and efforts to support the Hague conventions.

The sudden outbreak of the First World War in July 1914 dismayed the peace movement. Socialist parties in every industrial nation had committed themselves to antiwar policies, but when the war came, all of them, except in Russia and the United States, supported their own governments. There were highly publicized dissidents, some of whom were imprisoned for opposing draft laws, such as Eugene Debs in the U.S. In Britain, the prominent activist Stephen Henry Hobhouse was jailed for refusing military service, citing his convictions as a "socialist and a Christian". Many socialist groups and movements were antimilitarist, arguing that war by its nature was a type of governmental coercion of the working class for the benefit of capitalist elites. The French socialist pacifist leader Jean Jaurès was assassinated by a nationalist fanatic on 31 July 1914. The national parties in the Second International increasingly supported their respective nations in war, and the International was dissolved in 1916.

In 1915, the League of Nations Society was formed by British liberal leaders to promote a strong international organisation that could enforce the peaceful resolution of conflict. Later that year, the League to Enforce Peace was established in the U.S. to promote similar goals. Hamilton Holt published a 28 September 1914, editorial in his magazine the Independent called "The Way to Disarm: A Practical Proposal" that called for an international organization to agree upon the arbitration of disputes and to guarantee the territorial integrity of its members by maintaining military forces sufficient to defeat those of any non-member. The ensuing debate among prominent internationalists modified Holt's plan to align it more closely with proposals offered in Great Britain by Viscount James Bryce, a former British ambassador to the United States. These and other initiatives were pivotal in the change in attitudes that gave birth to the League of Nations after the war.

In addition to the traditional peace churches, some of the many groups that protested against the war were the Woman's Peace Party (which was organized in 1915 and led by noted reformer Jane Addams), the International Committee of Women for Permanent Peace (ICWPP) (also organized in 1915), the American Union Against Militarism, the Fellowship of Reconciliation and the American Friends Service Committee. Jeannette Rankin, the first woman elected to Congress, was another fierce advocate of pacifism, the only person to vote against American entrance into both wars.