The "King of Aragon" refers to the monarchs who ruled the historical region of Aragon in northeastern Spain. The Kingdom of Aragon was established in the 9th century and became a significant power during the Middle Ages, especially after the union with the Kingdom of Navarre and the marriage of Ferdinand II of Aragon to Isabella I of Castile, which led to the unification of Spain. The Aragonese monarchy played a crucial role in the Reconquista, the centuries-long effort to reclaim the Iberian Peninsula from Muslim rule. The kingdom was known for its maritime power and influence in the Mediterranean, particularly through its connection to the Crown of Aragon, which included territories like Sicily and Sardinia.