In his historical satirical novel Candide, Voltaire (Francois-Marie Arouet) presented a fanciful vision of the Jesuit missions established among the Guarani in parts of what today are Argentina, Paraguay, and Brazil. Some scholars have characterized the missions as having been a socialist utopia, or an independent republic located on the fringes of Spanish territory in South America. What was the reality? This study presents a detailed analysis of one of the Jesuit missions, Los Santos Martires del Japon, and the story of the creation of mission communities on a frontier contested by Spain and Portugal during the seventeenth and eighteenth centuries. It documents the historical realities of the Jesuit missions, their patterns of development, and the demographic consequences for the mission populations of military conflict.