 Berbers were thrown into the vanguard while Arab forces were kept in the back; they were assigned garrison duty on the more troubled frontiers).Although the Ifriqiyan Arab governor Musa ibn Nusair had cultivated his Berber lieutenants (most famously, Tariq ibn Ziyad), his successors, notably Yazid ibn Abi Muslim, had treated their Berber forces particularly poorly.Most grievously, Arab governors continued to levy extraordinary "dhimmi" taxation (the "jizyah" and "kharaj") and slave-tributes on non-Arab populations that had converted to Islam, in direct contravention of Islamic law.