ultimatum, 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 These provisions were superseded by the Covenant of the League of Nations and later by the Charter of the United Nations, which limited the right of states to use war as an instrument of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.

An ultimatum (the last one) is a demand whose fulfillment is requested in a specified period of time and which is backed up by a threat to be followed through in case of noncompliance. An ultimatum is generally the final demand in a series of requests. As such, the time allotted is usually short, and the request is understood not to be open to further negotiation.

The threat which backs up the ultimatum can vary depending on the demand in question and on the other circumstances. The following are commonly encountered:

• in diplomacy, the threat to declare war, to take other military action or to impose sanctions such as trade restrictions or embargoes.
• in a kidnapping, the threat by the kidnappers that they will kill the hostages if their demands are not met

In everyday life, ultimatums are also sometimes made in various contexts, such as in:

• a legal proceeding (e.g. the demand to strike a plea bargain or face a trial)
• a business relationship (e.g. the demand to accept a certain price or to forgo the deal)
• a labor dispute (e.g. the demand to grant a hike in salaries or another improvement in the employees' conditions, or face a strike)
• a personal relationship (e.g. the demand to desist in an extramarital affair or to face a divorce).

Such ultimatums may or may not be legal and/or socially acceptable. Ultimatums are also a regular part of the crime of extortion.