The Charter was designed to remedy certain constitutional defects and omissions in the Covenant that the founders of the UN believed had been partly responsible for the League's inability to halt the drift toward a second world war in the 1930s. These defects and omissions included the absence of any provision imposing a total ban on war, the provision of an overly rigid procedure for negotiating disputes between states, and the failure to vest the League's Council with sufficient powers to prevent the outbreak of hostilities or to terminate hostilities that had already begun.
The Covenant forbade military aggression but did not reject the limited right of a state to start a war, provided that it had first submitted the dispute to arbitration, judicial decision, or the Council of the League. If one party accepted the findings of the negotiating body and the second did not, the first might then resort to war legally after a "cooling-off" period.
The Charter recognizes no circumstances under which a nation may legally start a war. Article 51 does guarantee the right to individual or collective self-defense, which is a right to respond to an illegal armed attack but not to initiate one. If the Security Council decides that a "threat to the peace" exists, it has the power to order collective enforcement measures. These are mandatory for all member states and may include economic sanctions or military measures, but the power rarely has been invoked. (See the chapter on International Peace and Security.)