The Treaty of Tordesillas.

Though its consequences were ultimately indirect, the Treaty of Tordesillas between Spain and Portugal had an enormous effect on European warfare in North America during the sixteenth century. The agreement evolved out of the earlier Treaty of Alcacovas (1479), which had recognized exclusive Spanish and Portuguese spheres of interest in West Africa and the islands of the eastern Atlantic. Christopher Columbus's discovery of the New World in 1492 upset that arrangement, however, because the Alcacovas accord contained no provision for newly found territories. To resolve the dispute, Spain and Portugal asked for a ruling from Pope Alexander VI. The pontiff responded by issuing a papal bull—a formal directive from the Pope—that divided the non-Christian world into two spheres demarcated by a line running north to south through the Atlantic Ocean. Spain, according to the Pope's decree...