The Arab Conquest of Spain can be a bit of a tricky animal to figure out. Details surrounding the events in questions are hazy, as most of our sources are much later in origin and much more specific than our earlier accounts. The most reliable account is the Chronicle of 754, likely written in Toledo by a Mozarabic Christian living under Muslim rule and the description of the actual conquest is frustratingly brief and nondescript. However, there I some contemporary material evidence in the form of coinage bearing the names of two different Visigothic kings, Roderic and Achila, which combined with regnal lists, treaties, and other documentation immediately following the conquest, are useful in putting together a general account of what happened.

It would appear that the death of Wittiza prompted something of a succession crisis among the Visigothic elite. Roderic managed to come to power with the support of some of the aristocracy, but the situation was still unstable; in fact, there might have been other claimants who maintained power bases in other parts of the peninsula. Following their conquests of Ceuta and Tangiers in 705/706, Arab and Berber armies had begun raiding the southern coast of Spain. In 711/712, it seems that Roderic went south with an army and encountered a force led by the newly appointed governor of Tangiers, Tariq ibn Ziyad, who at some point in the past year had crossed into the peninsula. The Visigothic force was overwhelmed and Roderic either died in battle, or escaped and died shortly thereafter. Around the same time another army led by Musa ibn Nusayr, the governor of Ifriqiya and Tariq's superior, landed at Cadiz and began to make its way towards Toledo, where a claimant to the Visigothic throne might have emerged in the wake of Roderic's fall. Musa easily took the city and executed an unspecified number of the old Visigothic palatine aristocracy. The image we get from the Chronicle is one of regional warfare and chaos, but Musa was able to pacify the capital and the surrounding regions, and before he was called back to Damascus by the caliph at the end of 712, he and Tariq had managed to force the submission of most of the peninsula. Areas like the river valley of the Ebro and the northern mountainous regions took a little longer, but a series of treaties, likely made with individual cities, settlements, and nobles, served to restore social order and establish the authority of the caliphal governors in the peninsula.

So why did Visigoth rule collapse so quickly? While the combined armies of Tariq and Musa likely only numbered a few thousand, it is also just as likely that Roderic's army was not much larger. At this point, the Visigoths were probably unaccustomed to mobilizing large military forces beyond what was needed to keep the peace and pacify periodic raids. There is also an implication that Roderic's forces were divided in some way, or were in some way weakened by recent political instability; Roderic's reign was by no means secure. The subsequent death of the Visigothic king, the seizure of Toledo, and the elimination of a sizable portion of the palatine aristocracy effectively froze the Visigothic state. The Arab and Berber army may not have been large, but they effectively caught the Visigoths with their pants down and managed to capitalize on the chaos following the death of the king in an already politically divided society, striking quickly at the potential center of organized monarchical resistance.

The cooperation of local landlords was important in the consolidation of Arab rule in the peninsula. Treaties ensured, at least initially, continuity in local authority in return for certain concessions and taxes rendered to the conquerors. We are quite fortunate in that the text of one of these treaties has survived to the present day. Theodemir, the lord of seven towns in the south-east of the peninsula agreed to pay an annual tribute of one dinar per capita, along with four measures of wheat, four of barley, four jugs of grape juice, four of vinegar, two of honey, and two of oil, with a half payment for every slave. The inhabitants of Theodemir’s towns were also prohibited from giving refuge to the enemies of the conquerors. In return, they would be left unmolested, their lords would not be deposed, and they would be allowed to practice their religion in peace. The treaties served the purpose of pacifying large swathes of territory without having to garrison troops, allowing the conquerors to concentrate their numerically limited forces on problematic regions. It is likely that these treaties were common; Pamplona probably agreed to a similar one.

I would not go so far to say that the Arab Conquest experienced widespread support though. Other parts of the peninsula resisted. Merida in particular was subject to a length siege, and the conquest of Zaragoza saw a violent end as well. It is also likely that Cordoba put a brief resistance that ended bloodily. Achila, mentioned at the beginning of this post, might have held out as a ‘king’ in the northeast of the peninsula for a little while. The peoples in the North in what is now Asturias managed to elude Arab control. We also shouldn’t assume that aristocrats who agreed to treaties did so merrily, and weren’t instead submitting to the practical realities of their situation. The Visigoths, after all, ruled a diverse peninsula. Generally speaking, the Visigothic kings had managed to hold it all together (though the extent to which real monarchical authority was felt locally is up to debate). In fact, despite the turmoil of the succession crisis prior to the conquest, there does not appear to have been any threat to the territorial integrity of the kingdom. The rapid conquest of 711/712 effectively eliminated the possibility of a centralized and organized resistance, and the peninsula quickly fragmented along the lines of pre-existing fractures. Resistance and surrender were haphazard and locally negotiated.

The Visigoths may have not been the same type of Christian as the Iberians, but they were at least Christian. Islam was entirely foreign… much of the Iberian Christian population just converted to Islam and integrated into the Caliphate.

I do want to briefly address this point. The Visigothic aristocracy had begun the process of converting to Catholicism during the reign of Reccared in 587, and by the time of the conquest, the kingdom was thoroughly Catholic. The conversion of the peninsula to Islam was also a slow process, as it was elsewhere in the Caliphate. The pendulum probably did not swing in favor of Islam until the tenth or perhaps even the eleventh century.

I hope that answers most of your questions. If you have any follow-ups or clarifications, please feel free to ask.