The Zimmerwald Conference was held in Zimmerwald, Switzerland, from September 5 to 8, 1915. It was the first of three international socialist conferences convened by anti-militarist socialist parties from countries that were originally neutral during World War I. The individuals and organizations participating in this and subsequent conferences held at Kienthal and Stockholm are known jointly as the Zimmerwald movement.

Antimilitarism is a doctrine that opposes war, relying heavily on a critical theory of imperialism and was an explicit goal of the First and Second International. Whereas pacifism is the doctrine that disputes should be settled without recourse to violence, Paul B. Miller defines anti-militarism as "ideology and activities...aimed at reducing the civil power of the military and ultimately, preventing international war". Cynthia Cockburn defines an anti-militarist movement as one opposed to "military rule, high military expenditure or the imposition of foreign bases in their country". Martin Ceadel points out that anti-militarism is sometimes equated with pacificism—general opposition to war or violence, except in cases where force is deemed necessary to advance the cause of peace.