This chapter examines the development of the law of armed conflict to limit and prohibit all use of armed force in international relations, beginning with the Hague Convention of 1907 for the Pacific Settlement of Disputes, the League of Nations, the Locarno agreement, the Pact of Paris, and the United Nations Charter. It discusses the effect and influence of the Just War theory on the contemporary international legal system in general and the United Nations Charter in particular, as regards both jus ad bellum and jus in bello. The Covenant of the League of Nations was a product of the desire of States to find a way to prevent a recurrence of the First World War. The Assembly determined that the League Council could decide that a violation had occurred and recommend enforcement measures that should be taken. The 1925 Locarno agreement was another important step towards prohibiting initiation of war unless in exceptional cases.