Final answer:
The unification of Spain under Isabella's desire for unity, through her marriage to Ferdinand and the completion of the Reconquista, led to a stronger centralized royal authority, internal economic and cultural flourishing, and set the foundation for Spain's golden age of exploration and colonial expansion.
Step-by-step explanation:
Isabella's desire for unity had a profound impact on Spain, fundamentally shaping its future as a nation. The marriage of King Ferdinand of Aragon and Queen Isabella of Castile in 1469 unified two of the strongest kingdoms on the Iberian Peninsula and initiated the consolidation of Spain into a single, Catholic nation. This unification was solidified further by the completion of the Reconquista in 1492, with the conquest of Granada, marking the end of Muslim rule in Spain and beginning a period of religious homogenization centered on Catholicism. Additionally, their rule marked the start of an era of exploration and conquest, exemplified by their support for Christopher Columbus's voyage, which expanded Spain's horizons and led to the creation of a vast overseas empire.
The effects of this unity were both internal and external. Domestically, they lessened the power of the nobles to strengthen royal authority, increased commerce and industry, influenced cultural development through the building of great cathedrals and churches, and the monarchy bolstered the power of the church and religious institutions. Internationally, the unification of Spain under Ferdinand and Isabella positioned the country to embark on extensive exploration and conquest, fundamentally altering Spain's standing in the world and setting the stage for its golden age of global influence and power.