The invasion of Spain wasn't something planned by the Arabs but was born because of political opportunism, although the conquest is very poorly documented with the sources being either in contradiction or being too vague which lead to more questions than answer.

In the Chronicle of the 754, its stated that after the king Witiza died in 710 the throne got usurped by a nobleman named Roderic under the instigation of the senate. Then this new king assembled an army to fight the Arab and Berber sent by Musa ibn Nusayr under Tariq ibn Ziyad. This fact is in the contradiction because in the muslim sources it is stated that it was Tariq ibn Ziyad own initiative to invade Iberia and Musa is portrayed initially as dissatisfied with Tariq because he didn’t got consulted, but was quickly won over once he understand the profitability of the invasion. Roderic lost to the Arabs in the battle and then die. The Christian sources, both the Chronicle of the 754 and the Chronicle of Alphonse III, stated that Roderic was betrayed by some nobles during the battle.

Some theories stated that Tariq reasoning of invasion was because of the political instability in the peninsula. Some numismatic evidence suggest that Spain was divided in two part because in the southern and central part coins under Roderic's name are found, meanwhile in the northern part those under the name of certain Achila are encountered. Two continuation of the Chronicon Regum Visigothorum record Achila's three year reign following immediately witiza's. So Tariq wanted to take the advantage of this political fragmentation and he started the Arab invasion.

However it's suggested that Tariq ibn Ziyad expedition in Spain was initially just a continuation of large scale raids in Iberia, which were common in the pre islamic period, and only the death of Roderic, which created a power vaccum, lead to the start of the conquest, in particularly after Musa ibn Nusayr joined the Berber general the following year.

Another problem with the sources is the figure of Julian who in the muslim sources appear as a key figure for the invasion of Spain, meanwhile in the chronicle of 754 no evidence about him is found. Its stated that Julian, the lord of Ceuta, after knowing that Roderic raped his daughter, decided to take revenge by allying with the arabs and helping them to cross the strait. There is a quite discussion about this character, whether he is real or not. Collins, in his book The Arab conquest of Spain, argue that Julian didn't existed as he didn't appear in the chronicle of 754, the earliest sources on the invasion and was reliable because it was written during the Arab rule. However Chalmeta, in his work invasion and islamizaciòn, suggest that Julian was real and he lied about his daughter rape in order to justify his betrayal to Roderic. The concept of betrayal also return in the Chronicle of Alphonse III in which the sons of Witiza, after losing their throne, asked help from arabs to take back the lost reign from Roderick and guide them.