The "King of Aragon" refers to the monarchs who ruled the historical Kingdom of Aragon, located in northeastern Spain. This kingdom was established in the early Middle Ages and became a significant political entity in the region. The Aragonese monarchy played a crucial role in the Reconquista, the effort to reclaim the Iberian Peninsula from Muslim rule. The Kingdom of Aragon eventually merged with the Kingdom of Castile through the marriage of Ferdinand II of Aragon and Isabella I of Castile in 1469. This union laid the foundation for the modern nation of Spain. The Aragonese crown was influential in European politics, particularly during the late medieval period.