"This dissertation presents an historical analysis of the relations between the Spanish and Portuguese along the frontier of their imperial possessions in East and Southeast Asia, and the Southwest Pacific, from the middle of the sixteenth to the end of the seventeenth century. This political frontier was created as the antemeridian of the Atlantic line of demarcation initially adjudicated by the papacy in 1493, and subsequently modified by the Treaty of Tordesillas in 1494. The incapacity of sixteenth- and seventeenth-century cosmographers and cartographers to accurately define and depict this boundary would give rise to ongoing tensions between the Spanish and Portuguese, both on the fringes of their respective empires, and at their courtly centers. Matters were further complicated in 1580, when Philip II of Spain acquired the throne of Portugal. From 1580-1640, the Spanish Habsburgs ruled as lords of two theoretically distinct empires - that of Spain and Portugal. This study explores the complexities of governance that resulted from this union of crowns. In this analysis of frontier rivalries and cooperation, this dissertation has adopted methodological approaches from intellectual and political history. It also uses methods appropriate to a History of Science analysis of the roles of nautical science, cartography, and cosmology in the formulation of Portuguese and Spanish imperial strategies. It employs a comparative method to investigate and describe the complex circumstances surrounding Luso-Spanish conflict and cooperation. Unpublished primary materials found principally in Spanish and Portuguese archives have been used extensively throughout. This material has been contextualized and supplemented by published primary and secondary sources. The types of source material investigated include cartographic examples, printed tracts on cosmology, geography, theology, and travel narratives. Reports written by frontier officials, memorials requesting royal favor or assistance, and documentation produced by royal advisory councils have also provided crucial evidence for this comparative history. The analysis of these sources, using the methodologies noted above, has resulted in a new history of Iberian Asia. Spanish and Portuguese overseas expansion was built on a complex set of economic, political, and spiritual motives. Some of these display an extraordinary continuity, but little has been done up to this time in exploring these continuities in terms of the Habsburg imperial policies in Asia and the Pacific. This dissertation argues, for example, that the ideals that enlivened the imperial project of Dom Manuel of Portugal or the speculative cosmology of Christopher Columbus also came to play a significant role in the development of creative imperial strategies at the beginning of the reign of Philip III in the early seventeenth century. In so doing, this dissertation revises the image of Philip as a disengaged monarch, ruling over an empire caught in the grip of inevitable decline. Faced with the mounting threat of Dutch expansion in the waters of East and Southeast Asia, Philip sought new ways of guaranteeing the security of Iberian possessions in the region. The projects brought before the Crown by the Portuguese navigator, Pedro Fernandez de Quiros, become symbolic of this new age in imperial strategy. Quiros possessed an eclectic mix of talents and interests. His technical mastery in navigation, his invention of scientific instruments, his study of cosmology, and his expertise in mapmaking became well known at the papal and Spanish courts in the early seventeenth century. His original proposals for voyages of exploration also included a series of scientific experiments aimed at improving contemporary techniques in navigation and measurement. He combined these interests with a lofty vision of Iberian imperial destiny. In his extensive writings there are also hints of the millenarian ideas of the medieval Abbot Joachim of Fiore, mediated through the Franciscan renewal of the late Middle Ages. This intellectual inheritance became focused in Quiros's vision of a new Spanish society to be founded in the Southwest Pacific. That such ideas were entertained seriously at the Court of Philip III suggests an interest on the part of the Habsburg Crown with forging a new strategy aimed at revising the existing policy of a strict separation between the Spanish and Portuguese empires. It further suggests the formulation of policies for integrating the maintenance of the Crown's existing possessions in Asia with exploration and discovery in the Pacific."--Leaves 1-3.