It is interesting that in earlier times, both sides for quite a long time refused to recognize this as a struggle between religions—that is, to recognize the other as a rival universal religion. They saw it rather as between religion—meaning their own true faith—and the unbelievers or infidels (in Arabic, kafir). Both sides long preferred to name each other by non-religious terms. The Christian world called the Muslims Moors, Saracens, Tatars, and Turks; even a convert was said to have “turned Turk.” The Muslims for their part called those in the Christian world Romans, Franks, Slavs, and the like. It was only slowly and reluctantly that they began to give each other
religious designations, and then these were for the most part inaccurate and demeaning. In the West, it was customary to call Muslims Mohammadans, which they never called themselves; this was based on the totally false assumption that Muslims worship Muhammad in the way that Christians worship Christ. The usual Muslim term for Christians was Nazarene—nasrani—implying the local cult of a place called Nazareth.
The declaration of war came almost at the very beginning of Islam. According to an early story, in the year 7 of the Hegira, corresponding to 628 C.E., the Prophet sent six messengers, with letters, to the Byzantine and Persian emperors, the Negus of Ethiopia, and other rulers and princes, informing them of his advent and summoning them to embrace his faith or suffer the consequences. The authenticity of these prophetic letters is doubted, but their message is accurate in the sense that it does reflect a view dominant among Muslims since early times.
A little later we have hard evidence—and I mean hard in the most literal sense—in inscriptions. One of the famous sights of Jerusalem is a remarkable building known as the Dome of the Rock. It is in several ways significant. It is built on the Temple Mount, a place sacred to the Judeo-Christian tradition. Its architectural style is that of the earliest Christian churches. The oldest Muslim religious building outside Arabia, it dates from the end of the seventh century and was built by Abd al-Malik, one of the early caliphs. Specially significant is the message in the inscriptions on the building: “He is God, He is one, He has no companion, He does not beget, He is not begotten” (Qur’an, IX, 31-3; CXII, 1-3). This is clearly a direct challenge to certain central principles of the Christian faith.
Interestingly , the caliph proclaimed the same message with a new gold coinage. Until then, striking gold coins had been an exclusive Roman, later Byzantine prerogative, and other states, including the Islamic caliphate, imported them as required. The Islamic caliph for the first time struck gold coins, breaching the immemorial privilege of Rome, and putting the same inscription on them. The Byzantine Emperor understood the double challenge, and went to war— without effect.
The Muslim attack on Christendom and the resulting conflict, which arose more from their resemblances than from their differences, has so far gone through three phases. The first dates from the very beginning of Islam, when the new faith spilled out of the Arabian Peninsula, where it was born, into the Middle East and beyond. It was then that Muslim armies from Arabia conquered Syria, Palestine, Egypt, and North Africa—all at that time part of the Christian world—and began the process of Islamization and Arabization. From there they advanced into Europe, conquering Spain, Portugal, Sicily, and the adjoining regions of mainland southern Italy, all of which became part of the Islamic world, and even crossing the Pyrenees and for a while occupying parts of France.