International arbitration may be defined as the settlement of a difference between states through the decision of one or more individuals or a tribunal or court chosen by the parties to the dispute. An arbitrator may be the chief of state of a nation not concerned with the dispute; an ambassador, minister, or other official; or even a private individual. When a monarch or a president is an arbitrator he usually does not act personally; indeed, he delegates most responsibilities to the appropriate legal authorities of his government. When the parties to an arbitration decide to establish a tribunal, they may choose judges from their own nationals and then agree upon another individual to act as umpire. Sometimes they ask the head of another government to choose an umpire or leave the choice of umpire to the arbitrators already appointed. In several nineteenth-century cases no individuals were designated as umpires. Arbitrations may be concerned with questions of international law or facts. When arbitrations are primarily concerned with facts, as in pecuniary claims or boundary cases, the group of arbitrators is generally called a commission, but no precise distinction can be drawn between commissions and tribunals. An arbitral decision is called an award, and it may be set aside if there are reasons to believe that it was not given in good faith or was not in accord with international law or the preliminary special agreement, usually called a compromis, concluded by the parties to the arbitration.
Historians and anthropologists have discovered arbitral customs and institutions in many cultures. The city-states of ancient Greece developed fairly elaborate arbitral procedure; on occasion they organized groups of arbitrators similar to modern international tribunals. During the Middle Ages, popes, princes, jurisconsults, and even city governments acted as arbitrators. Arbitration was less important during late medieval and early modern times, but it never disappeared altogether from international relations. Occasionally, European governments made use of it when trying to resolve American questions. In fact, some aspects of the first problem in the diplomatic history of the European conquest of the Western Hemisphere—the location of the dividing line between Spanish and Portuguese interests—suggest later arbitral practices. When Portugal challenged Spain's rights in the lands Columbus had discovered, King Ferdinand asked Pope Alexander VI to confirm the Spanish title. The pontiff obliged, issuing in 1493 a series of bulls in which he drew a line between the imperial claims of the two countries. The Portuguese protested the papal decision, and in 1494, Spain and Portugal, in the Treaty of Tordesillas, moved the line westward and agreed that a commission of surveyors and mariners should locate the line. While the two governments never set up the commission, the provisions of the treaty calling for such a body are evidence that commissions were of some importance in international relations at that time.
Commissions appeared occasionally in connection with England's colonial problems during the seventeenth and eighteenth centuries. The Treaty of Westminster, which Oliver Cromwell concluded with the Dutch at the end of the First Anglo-Dutch War in 1654, referred claims concerning the East Indies and the Americas to a commission. Apparently this commission met but failed to arrive at a decision. England and France in 1686 referred disputes over American matters to a commission, but it disbanded after outbreak of the War of the League of Augsburg. The Anglo-French treaties of Ryswick in 1697 and Utrecht in 1713 and the Treaty of Seville concluded by Great Britain, France, and Spain in 1729 provided for commissions to deal with American problems. All failed. After the War of the Austrian Succession, Britain and France established a commission for American questions. Again, failure. Certainly, the performances of commissions during the colonial era should have encouraged no one to believe that arbitration would be of large importance in later American history, yet that series of failures kept the idea alive. After the United States won independence, there were many problems that American and British diplomats found difficult to settle through negotiation, and they turned to commissions almost as a matter of course.