Christopher Columbus returned to Spain from his epochal voyage to find his homeland’s standing in the world transformed. Indeed, the Genoese explorer’s discoveries had only inflamed an already-smoldering conflict between the Spanish monarchs, Ferdinand and Isabella, and Portugal's King João II: Which naval power was to control the world's oceans?

In an attempt to keep the peace, Pope Alexander VI—the notorious Rodrigo Borgia—issued the Inter Caetera, a papal bull that laid the foundation for the Treaty of Tordesillas. This edict of “the most famously cunning and corrupt of the Renaissance popes” created an imaginary line in the Atlantic Ocean, from pole to pole, dividing the entire known (and unknown) world between Spain and Portugal. Portugal was granted dominion over all territory east of the line, and all territory to the west was to be Spanish.

Just as the world's oceans were opened by Columbus' voyage of discovery, the treaty sought to limit control of the seas to these two favored Catholic nations. As Portugal reaped the bounty of the East Indies and the Spice Islands, Spain enriched itself with resources from the new lands to the west. The edict was to have a profound influence on world history: it propelled Spain and Portugal to superpower status and became the central grievance in more than 200 years of international espionage, piracy and warfare. Not until the next century, as legal scholar Hugo Grotius’ “Free Seas” theory gained acceptance, did New World exploration open up to the Dutch, the English and others.

Columbus’ expedition was the culmination of 16 years of relentless lobbying; the Genoan sought funding from Portuguese royalty—and was decisively rebuffed—many years before the charismatic, intellectually curious Queen Isabella agreed to bankroll the explorer’s “Enterprise of the Indies.” Though the vainglorious Columbus’ promises of a rapid route to the Spiceries (and copious quantities of gold) came to naught, the dramatic expansion of the known world made possible the pope’s audacious act of global diplomacy.

At the heart of one of the greatest international diplomatic and geopolitical agreements of the last five centuries were the strained relationships and marital machinations of a handful of powerful individuals. They were linked by a shared history, mutual animosity and personal obligations. “Pride, passion, enmity and petty quarrels between this privileged and powerful clique,” writes Bown, “stimulated and enflamed by Columbus’s hubris, led to a simmering, centuries-long global conflict that stemmed from the pope dividing the world in half in 1494.” Bown’s work compellingly blends late-medieval political intrigue with the thrill of exploration.