ultimatum (ŭl´tĬmā´təm), in international law, final, definitive terms submitted by one disputant nation to the other for immediate acceptance or rejection. Since refusal to accept the terms may lead to war or hostile measures, an ultimatum usually constitutes a conditional declaration of war. An ultimatum is written and indicates how its nonacceptance will be regarded. When a brief time limit is imposed, the crisis becomes more intense, because there is less opportunity for mediation or arbitration. The contracting powers at the second Hague Conference (1907) agreed to begin hostilities only after giving warning. These provisions were superseded by the Covenant of the League of Nations and later by the Charter of the United Nations, which limited the right of states to use war as an instrument of national policy. An ultimatum presented by Austria to Serbia on July 23, 1914, was the immediate cause of World War I. Hitler also presented several ultimatums (to Czechoslovakia and Poland) in the year before the outbreak of World War II. Japan, however, began its war with the United States with an attack rather than an ultimatum.