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.

The Second International (1889–1916) was an organisation of socialist and labour parties, formed on 14 July 1889 at two simultaneous Paris meetings in which delegations from twenty countries participated. The Second International continued the work of the dissolved First International, though excluding the powerful anarcho-syndicalist movement. While the international had initially declared its opposition to all warfare between European powers, most of the major European parties ultimately chose to support their respective states in World War I. After splitting into pro-Allied, pro-Central Powers, and antimilitarist factions, the international ceased to function. After the war, the remaining factions of the international went on to found the Labour and Socialist International, the International Working Union of Socialist Parties, and the Communist International.