The Spanish galleon system’s last hurrah occurred during the War of the Spanish Succession (1701-1714). A century’s worth of pirate and corsair attacks, budget shortfalls, fraud, and contraband trade had progressively weakened the Spanish crown’s transatlantic trade monopoly. The famous Portobello fairs linking Peru to Spain grew cumbersome by the end of Habsburg rule, and Portobello itself became harder to defend in the face of British aggression. Into this precarious situation stepped an array of French merchants, who seized the opportunity of Bourbon alliances to introduce foreign manufactures to Spanish South America in exchange for silver, the precious lubricant of global trade. They did so via Cape Horn, bypassing Panama and penetrating the Pacific by rounding the southern tip of South America. This dissertation examines the shared history that linked merchant communities between Peru and France between Bourbon accession and the end of the Seven Years' War, when trade policies changed. The merchants of Lima and Saint-Malo stand out for developing an intimate yet dangerous relationship that lasted from just before 1700 to the early 1760s. In their long and tangled affair, Limeños and Malouins tested one another in ways that exposed the changing nature of mercantile societies. Urged on or pulled back by their respective governing dynasties, city-based merchant families and associations on opposite sides of the globe deployed creative strategies to win profit from high-risk, ultra- long-distance trade. Along the way, enemies became friends and vice versa until the “long route” to Peru proved politically untenable. “Dangerous Liaisons” sheds light on this often-forgotten interlude in world economic history.