Clashes and raids on bordering Andalusian lands did not keep the Christian kingdoms from battling among themselves or allying with Muslim kings.[18] Some Muslim kings had Christian-born wives or mothers.[citation needed] Some Christian mercenaries, like El Cid, were contracted by taifa kings to fight against their neighbours.[18] Indeed, El Cid's first battle experience was gained fighting for a Muslim state against a Christian state.[citation needed] At the Battle of Graus in 1063, he and other Castilians fought on the side of al-Muqtadir, Muslim sultan of Zaragoza, against the forces of Ramiro I of Aragon.[citation needed] There is even an instance of a crusade being declared against another Christian king in Hispania.[72] Although Christian rulers Fernán González of Castile and Ramiro II of León had cooperated to defeat the Muslims at the Battle of Simancas (939), Fernán attacked Ramiro soon after and the Leonese–Castilian war that followed lasted until Ramiro's victory in 944.[73] Ramiro II's death caused the war of the Leonese succession (951–956) between his sons, and the winner Ordoño III of León concluded peace with caliph Abd al-Rahman III of Córdoba.[73]

A map of Christian realms in the north and Islamic taifas in the south (1037). During the Reconquista, the Iberian states not only fought along religious lines, but also amongst themselves and internally, especially during wars of succession and clan feuds.

After the defeat of Alfonso VIII, King of Castile, at Alarcos, Kings Alfonso IX of Leon and Sancho VII of Navarre entered an alliance with the Almohads and invaded Castile in 1196.[citation needed] By the end of the year Sancho VII had dropped out of the war under Papal pressure. Early in 1197, at the request of Sancho I, King of Portugal, Pope Celestine III declared a crusade against Alfonso IX and released his subjects from their responsibilities to the king, declaring that "the men of his realm shall be absolved from their fidelity and his dominion by authority of the apostolic see."[72] Together the Kings of Portugal, Castile, and Aragon invaded Leon. In the face of this onslaught combined with pressure from the Pope, Alfonso IX was finally forced to sue for peace in October 1197.[citation needed]

In the late years of Al-Andalus, Castile had the might to conquer the remnants of the kingdom of Granada, but the kings preferred to wait and claim the tribute of the Muslim parias. The trade of Granadan goods and the parias were a major means by which African gold entered medieval Europe.[citation needed]

Similarly, there was frequent Muslim infighting throughout the existence of al-Andalus. The Abbasid Revolution (747–750) divided Muslim rulers in Iberia into the pro-Abbasid Caliphate faction (based in Baghdad) and the pro-Umayyad faction (reconstituted as the Emirate of Córdoba).[74] Charlemagne's failed 778 campaign into Iberia was prompted by the invitation of the pro-Abbasid governor of Barcelona, Sulayman al-Arabi, which led to a brief Abbasid-Carolingian Alliance against the Umayyads.[74] During the Fitna of al-Andalus (1009–1031), the Umayyad-run Caliphate of Córdoba fell apart into rival taifas headed by Islamic emirs warring each other.[75] After the Christian king of Castile and León conquered Toledo in 1085, the emirs requested Yusuf ibn Tashfin, leader of the strict Islamic Almoravid sect, to come to their defence, which he did at the Battle of Sagrajas (1086). However, Yusuf soon turned on the Muslim emirs of Spain, defeating them all and conquering their lands by 1091.[76] A similar scenario occurred in 1147–1157, when the Almoravid dynasty fell, a Second Taifas period happened, and the Muslim-controlled cities of al-Andalus were conquered by the new Almohad Caliphate.[77] The War of the Granada succession (1482–1492) took place after the deposition of emir Abu'l-Hasan Ali of Granada by his son Muhammad XII of Granada; the deposed emir's brother Muhammad XIII of Granada also joined the fight. This succession conflict took place simultaneously with the Granada War, and was ended only by the Castilian conquest in 1492.[78]