The crown of Aragon and the crown of Castile agree in the Treaty of Almizra on the limits of their respective expansion into al-Andalus.

The Crown of Aragon was a composite monarchy ruled by one king, originated by the dynastic union of the Kingdom of Aragon and the County of Barcelona and ended as a consequence of the War of the Spanish Succession. At the height of its power in the 14th and 15th centuries, the Crown of Aragon was a thalassocracy controlling a large portion of present-day eastern Iberian Peninsula, parts of what is now southern France, and a Mediterranean empire which included the Balearic Islands, Sicily, Corsica, Sardinia, Malta, Southern Italy, and parts of Greece.

The Crown of Castile was a medieval polity in the Iberian Peninsula that formed in 1230 as a result of the third and definitive union of the crowns and, some decades later, the parliaments of the kingdoms of Castile and León upon the accession of the then Castilian king, Ferdinand III, to the vacant Leonese throne. It continued to exist as a separate entity after the personal union in 1469 of the crowns of Castile and Aragon with the marriage of the Catholic Monarchs up to the promulgation of the Nueva Planta decrees by Philip V in 1716.

The Treaty of Almizra was the third of a series of three treaties between the Crown of Aragon and Crown of Castile meant to determine the limits of their expansion into Andalusia so as to prevent squabbling between the Christian princes. Specifically, it defined the borders of the Kingdom of Valencia. James I of Aragon signed it on 26 March 1244, but Alfonso X of Castile did not affirm it until much later. According to the treaty, all lands south of a line from Biar to Villajoyosa through Busot were reserved for Castile. This ended most further Aragonese expansion on the Iberian peninsula.

Al-Andalus was the Muslim-ruled area of the Iberian Peninsula. The name refers to the different Muslim states that controlled these territories at various times between 711 and 1492. At its greatest geographical extent, it occupied most of the peninsula as well as Septimania under Umayyad rule. These boundaries changed through a series of conquests Western historiography has traditionally characterized as the Reconquista, eventually shrinking to the south and finally to the Emirate of Granada.