This article examines the declining prestige and utility of one of the mainstays of pre-Enlightenment peacemaking: treaties uniting once belligerent dynasties through marriage. By the late Middle Ages, interdynastic marriages had become such a common feature of the diplomatic landscape that the practice seemed almost transhistorical, something that was done always and everywhere. By the reign of Louis XIII, however, statesmen began stressing the limits of interdynastic marriage as a diplomatic strategy. This transformation of French affairs of state coincided with the appearance of prose romances that scholars identify retrospectively as the first French novels. This essay focuses on two of the most popular and influential of these novels, César Vichard de Saint-Réal's Dom Carlos (1672) and the Countess of La Fayette's La Princesse de Clèves (1678), as responses to the devaluation of the marriage diplomacy that had established dynastic networks between France and the Mediterranean world for centuries. Both works center on the 1559 Peace of Cateau-Cambrésis between France and Spain, which married Henri II's daughter Elisabeth de Valois to King Philip II. While Saint-Réal does not condemn such arrangements wholesale, he suggests that their success is highly subject to fortune. La Princesse de Clèves owes much to Saint-Réal's portrayal of the personal costs of marriages of state, as well as to Madame de La Fayette's own observations of the French and Savoyard courts. Although her focus on the interior life of her heroine limits explicit political commentary, her allusions to miserably unhappy queens-consort cast marriage diplomacy in an overwhelmingly negative light.