Final answer:
Ferdinand and Isabella united their forces against the Muslim presence, particularly the Moors in the Kingdom of Granada and other Islamic strongholds in Spain, in order to complete the Reconquista and spread Christianity.
Step-by-step explanation:
Ferdinand and Isabella united their forces against the Muslim presence, particularly the Moors in the Kingdom of Granada and other Islamic strongholds in Spain, in order to complete the Reconquista and drive out the Muslims from the Iberian Peninsula. They wanted to spread Christianity and combat the spread of Islam, which they saw as a threat to Roman Catholicism. Additionally, they funded Christopher Columbus' exploration of a westward route to Asia as a means to compete with the Portuguese and expand their empire.