By the 9th century Lyonesse and Spain had already been fighting for control of Aquitaine for hundreds of years, but the introduction of Islam increased the tension. Although King Alfonso of Spain converted in the year 860, the religion was slower to take hold among the common people - giving the Papacy and the Holy Roman Empire a casus belli to invade. In 872 the Franks entered Spanish territory in an attempt to overthrow Alfonso and impose a Christian king. They were defeated at the Battle of Cadurcorum later that year, but the seeds of a 500 year enmity had been sown.