After Roderic came to power in Spain, Julian had, as was the custom, sent his daughter to the court of the Visigothic king to receive an education. It is said that Roderic raped her, and that Julian was so incensed he resolved to have the Arabs bring down the Visigothic kingdom. Accordingly he entered into a treaty with Tariq to secretly convey the Muslim army across the Straits of Gibraltar, as he owned a number of merchant ships and had his own forts on the Spanish mainland.
About April 29 711, the army of Tariq, composed of recent converts to Islam, was landed at Gibralter by Julian.
Tariq's army contained about 7,000 men, and Musa is said to have sent an additional 5.000 reinforcements.Roderic, to meet the threat, assembled an army said to number 100,000.Most of the army was commanded by, and loyal to, the sons of Wittza, whom Roderic had brutally deposed. Tariq won a decisive victory when Roderic was defeated and killed on July 19 at the Battle of Guadalete.
On the advice of Julian, Tariq split his army into various divisions which went on to capture Cordoba, Granada and other places, while he remained at the head of the division which captured Toledo and Guadalajara . Tariq was de facto governor of Hispania until the arrival of Musa a year later.
Both Tariq and Musa were simultaneously ordered back to Damascus by the Umayyad Caliph Umayyad Caliph Al-Walid in 714, where they spent the rest of their lives.