In 711, 17.000 horse riders Berbers crossed the strait to Tarifa, Spain. First, they did not did it as a whole. They did not had enough boats. I suppose it took them quite a long time to unite this battalion, not even an army. Then, in ten years, their conquered most of Spain. It can't have happened without the collaboration of the local population. That's when I remembered other researches of mine, about slavery in Rome. For sure, Spanish colonies, which were a "reflection and image of Rome", were doing large use of slaves. What if it was the slave population in Spain, seeing in Islam a way to liberate themselves from their former masters, that helped and supported Berbers on their way to the North.