The death of Castile’s heir Juan unexpectedly opened the path to Iberian dynastic union.

Evidence

Portugal and Castile, each engaged in bold overseas ventures, had, in the final decade of the fifteenth century, a firm and successful policy of avoiding conflict between each other within the Iberian Peninsula itself. Matrimonial alliances sought to cement and reinforce good relations, but these were bonds from which there can have been no immediate expectation that an actual political merger of the kingdoms would result. That was certainly still the situation in 1497 when Manuel the Fortunate (who had acceded to the Portuguese throne in 1495) asked for the hand of Isabel, one of the daughters of the Catholic Monarchs. There was already a male heir to the Castilian throne, Juan, when Manuel married Isabel, so there was at the time no reason to suppose that this match would lead to the political union of Portugal and the Spanish kingdoms. However, the young Castilian prince was to die that same year altogether unexpectedly, and so it did seem for a time likely that any son of Manuel and Isabel (she, with some reluctance as we will see, had accepted the proposal of marriage) would eventually rule over almost all the peninsula. — L. P. Harvey, Muslims in Spain, 1500 to 1614, 107

Categorization

• XREF: Connects to the broader pattern of dynastic accidents shaping empires, similar to how chance deaths determined succession crises in other royal houses discussed elsewhere in European history.