Isabella I (Spanish: Isabel, 22 April 1451 - 26 November 1504) reigned as Queen of Castile from 1474 until her death. Her marriage to Ferdinand II of Aragon became the basis for the political unification of Spain under their grandson, Charles I. After a struggle to claim her right to the throne, she reorganized the governmental system, brought the crime rate to the lowest it had been in years, and unburdened the kingdom of the enormous debt her brother had left behind.

At the end of the Reconquista, only Granada was left for Isabella and Ferdinand to conquer. The Emirate of Granada had been held by the Muslim Nasrid dynasty since the mid-13th century. Protected by natural barriers and fortified towns, it had withstood the long process of the reconquista. On 1 February 1482, the king and queen reached Medina del Campo and this is generally considered the beginning of the war for Granada. While Isabella's and Ferdinand's involvement in the war was apparent from the start, Granada's leadership was divided and never able to present a united front. It still took ten years to conquer Granada, however, culminating in 1492.  The Spanish monarchs recruited soldiers from many European countries and improved their artillery with the latest and best cannons. Systematically, they proceeded to take the kingdom piece by piece. In 1485 they laid siege to Ronda, which surrendered after only a fortnight due to extensive bombardment. The following year, Loja was taken, and again Muhammad XII was captured and released. One year later, with the fall of Malaga, the western part of the Muslim Nasrid kingdom had fallen into Spanish hands. The eastern province succumbed after the fall of Baza in 1489. The siege of Granada began in the spring of 1491 and at the end of the year, Muhammad XII surrendered. On 2 January 1492 Isabella and Ferdinand entered Granada to receive the keys of the city, and the principal mosque was reconsecrated as a church. The Treaty of Granada was signed later that year, and in it Ferdinand and Isabella gave their word to allow the Muslims and Jews of Granada to live in peace. An uprising by the Moors in 1500 caused the Catholic side to consider that the Moors had violated the Treaty: this gave them a justification for revoking its provisions. See Morisco Revolt.  During the war, Isabella noted the abilities and energy of Gonzalo Fernandez de Cordoba and made him one of the two commissioners for the negotiations. Under her patronage, De Cordoba went on to an extraordinary military career that revolutionised the organisation and tactics of the emerging Spanish military, changing the nature of warfare and altering the European balance of power.

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Under her patronage, De Cordoba went on to an extraordinary military career that revolutionised the organisation