of the Byzantine Empire shifted from the western reconquests of Justinian to a primarily defensive position, against the Islamic armies on its eastern borders. Without Byzantine interference in the emerging Christian states of western Europe, the situation gave a huge stimulus to feudalism and economic self-sufficiency. The view of modern historians is that one of the most important effects was the strain it put on the relationship between Rome and Byzantium. While fighting for survival against the Islamic armies, the Empire was no longer able to provide the protection it had once offered to the Papacy; worse still, according to Thomas Woods, the Emperors "routinely intervened in the life of the Church in areas lying clearly beyond the state's competence". The Iconoclast controversy of the 8th and 9th centuries can be taken as a key factor "which drove the Latin Church into the arms of the Franks." Thus it has been argued that Charlemagne was an indirect product of Muhammad: "The Frankish Empire would probably never have existed without Islam, and Charlemagne without Mahomet would be inconceivable." The Holy Roman Empire of Charlemagne's successors would later come to the aid of the Byzantines under Louis II and during the Crusades, but relations between the two empires would be strained; based on the Salerno Chronicle, we know the Emperor Basil had sent an angry letter to his western counterpart, reprimanding him for usurping the title of emperor. He argued that the Frankish rulers were simple reges, and that each nation has its own title for the ruler, whereas the imperial title suited only the ruler of the Eastern Romans, Basil himself. Historiography and other sources Walter Emil Kaegi states that extant Arabic sources have been given much scholarly attention for issues of obscurities and contradictions. However, he points out that Byzantine sources are also problematic, such as the chronicles of Theophanes and Nicephorus and those written in Syriac, which are short and terse while the important question of their sources and their use of sources remains unresolved. Kaegi concludes that scholars must also subject the Byzantine tradition to critical scrutiny, as it "contains bias and cannot serve as an objective standard against which all Muslim sources may be confidently checked". Among the few Latin sources of interest are the 7th-century history of Fredegarius, and two 8th-century Spanish chronicles, all of which draw on some Byzantine and oriental historical traditions. As far