The development of the sacred right of peace
Initially, war was a legal and legitimate method to settle disputes (e.g. border disputes, invasion etc.) However, after the horrors of world war one, the International community began a more serious attempt to prevent war with the formation of the League of Nations. The League of Nations was established in the Covenant of the League of Nations. However, despite some success, the League of Nations ultimately failed after world war two broke out. After world war two, in stead of the league of nations, the United Nations was formed, with the United States at it's head.
The United Nations
The United Nations, formed after the atrocities of world war two, was governed by the UN Charter. The UN Charter, signed in June, 1945, explicitly made maintaining peace the primary purpose of the United Nations, similar to that of the League of Nations. Most importantly, the Charter gave the United Nations power to take measures to prevent and remove threats to peace. Thus, for the first time, war was (in effect) outlawed by the international community. It was in 1984, that the United Nations General Assembly determined the Declaration on the Right of Peoples to Peace. The Declaration proclaimed that 'peoples of our planet have a sacred right to peace'. It also declared that promoting and implementing this new right is a fundamental obligation of states.
Today the right of peace is balanced with the right of self-defence. Article 51 of the UN charter declares that 'states have an inherent right to individual or collective self-defence if an armed attack against them occurred'. Article 39 of the same charter allows the UN security council to authorise actions with the intention of maintaining or restoring peace. The ICC, or International Criminal Court, established in 2002, has the jurisdiction to try individual people for war crimes and other crimes against the international community.