The union between the Castilian and the Catalan-Aragonese monarchies was accomplished with the marriage of Isabella of Castile and Ferdinand of Aragon. It was a question, it should be borne in mind, that it was still a personal union, because, while giving it greater powers than those that had been granted to it in the marriage contract (1469), the agreement stipulated between the two spouses in 1474, at the time of their coronation as sovereign king of Castile, continued to place many limitations on Ferdinand’s authority as king of Castile. But in this way two results of enormous importance were achieved. First of all, during the life of the two spouses, the mutual limitations did not have serious consequences in practice; and then, waiting to become definitive with Charles I, the union was preserved even at the death of Isabella (November 26, 1504), because, by virtue of the deceased queen’s testamentary dispositions, Ferdinand immediately assumed the regency of Castile, held it until the end of June 1506, when he ceded power to his daughter Giovanna and her husband Philip of Austria, and soon took it back to the death of the latter (November 25, 1506), in the name of their son Carlo, to keep it until his death (January 23, 1516). And so, excluding the short period that elapsed between Isabella’s death and the beginning of Ferdinand’s second regency, when there was no lack of dissension between the father and daughter and the sharp contrast with Philip of Austria, it seemed that a ‘only mind governed the life of Spain,el Gran Capitán, the very skilled sailor Count Pedro Navarro, the Viceroy of Naples Ramón de Cardona, Cardinal Jiménez de Cisneros. On the other hand, the two monarchies, separated by centuries of history, by discordant interests, by an infinite number of prejudices, at least apparently preserved their independence and did not consider themselves inferior or subject to the other; so that they were able to collaborate in full agreement in the internal and foreign policies of their respective sovereigns and set out, even without knowing it, to reduce the differences that separated them and to give effective unity to their lives.

With the victories of Toro (1476) and of Albuera (1479) and with the peace of Alcoçobes (1479) the attempts of Alfonso V, who wanted to surround the crown of Castile, strengthened by the rights brought to him as a dowry by Giovanna la Beltraneja: so that Portugal’s aspirations for the neighboring state were forever undermined. Then, in 1481, two years after Fernando’s coronation as king of Aragon, on the death of his father John II (1479), the war against the Muslims of Granada was resumed, and, as had so often happened in the past, even now the two monarchies took part in the struggle with their united forces. Fortunate and unfortunate military enterprises intertwined with skilful negotiations and shrewd handling; the internal discords that broke out in the enemy camp between Sultan Muley Abū’l-Ḥasan, his son Boabdil (Abū ‛Abdallāh) and his brother Zaghal, who then assumed power, were made incurable, and to use the one against the other; Alora surrendered at the end of June 1484, Setenil in September, Ronda in May 1485, Loja in May 1486, Vélez-Málaga in April 1487, Málaga in the following August; Zaghal at the end of December 1489 ceded Baza, Cadiz, Almeria; and Grenade, defended by Boabdil and surrounded by siege in mid-1491, on January 6, 1492 saw the triumphal entry of the two Catholic kings within its walls who, each on his own account, represented their own states and at the same time Christian Spain, defeating of his age-old adversary.

Simultaneously and subsequently, the foreign policies of Ferdinand and Isabella and the results achieved in it were such as to satisfy the self-love of the various Spanish regions, to satisfy their particular interests, to reconcile the political antithetics hitherto followed by the two monarchies. Undoubtedly, at least in appearance, the one chosen was the traditional one of the Catalan-Aragonese monarchy, to which it gave, indeed, ever deeper and more extensive development. And it was anti-French policy, determined by ancient disagreements with the neighboring state for the dominance over the kingdom of Navarre – of which John II had been sovereign and which had subsequently passed to the Foix and Albrets – for the possession of Rosellón and Cerdaña – that Giovanni II had been forced to surrender to Louis XI, – for the Catalan-Aragonese expansion in Sicily and southern Italy – which since the war of the Sicilian Vespers had put France and Aragon against each other and which had increasingly separated souls after the conquest of Naples by Alfonso V and after the protection granted by France to the separatist revolution of Catalonia of 1462-72. Then, the enmity with the Capetian monarchy led to ever closer relations with its traditional rival, England, with which a treaty of offensive and defensive alliance was stipulated (1471-74), later confirmed by Catherine’s marriage., daughter of the Catholic kings, with the heir to the English throne, the future Henry VIII; and again led to be bound to the house of Austria with a double ties of kinship, that is, with the union of two other children of Ferdinand and Isabella, Giovanni Prince of Asturias and Giovanna, respectively with two children of Maximilian of Habsburg, Margaret and Philip.