In 1505, King Manuel I of Portugal (1495-1521) ordered the public printing of a letter officially addressed to Pope Julius II. In the letter, the Portuguese King defended his role as a champion of Christendom and scourge of Islam in the Indian Ocean. The most remarkable claim made by Manuel in this letter was that he was directly involved in persuading the Catholic monarchs of Spain Isabel of Castile and Fernando of Aragón to put an end to the toleration of Islam in Castile in 1501. This article focuses on this claim and whether or not it can merely be dismissed as the rhetoric of bombastic propaganda. It analyzes Luso-Spanish relations between 1495 and 1505 and highlights documentary evidence proving that Manuel did indeed put pressure on his Spanish neighbors to abolish the toleration of Islam during the tortuous negotiations surrounding his marriage to the Spanish princess Maria in 1501. Beyond assessing the historical significance of the letter, this article highlights the intricate connections between Portuguese imperial geopolitics and Iberian dynastic politics during this crucial period in the history of both the Spanish and Portuguese monarchies.