The Exchange of Princesses

Philippe d'Orlans, the regent of France, decides to marry 11-year-old Louis XV to the daughter of Philippe V of Spain, who is only four. Were Louis to die without begetting an heir, Orlans himself would finally be king. Orlans tosses his own daughter into the bargain, the 12-year-old Mademoiselle de Montpensier, who will marry the Prince of Asturias, the heir to the Spanish throne. The two nations trade their princesses in a grand ceremony in 1722, making bonds that should end the historical conflict; but nothing turns out as expected.