Folks, it’s all about jihad, from day one.
This concept of jihad as institutionalized holy war was first articulated and codified into Islam’s worldview by “warrior-theologians” (mujahidin-fuqaha) living and fighting along the Byzantine-Arab frontier (such as the mujahid Abdallah bin Mubarak, author of the seminal work Kitab al-Jihad or “Book of Jihad”).
The prevalent view was that, so long as Constantinople stood, the Cross would defy the Crescent. This is a literal point: Symbols played a great role in these wars. Less than a century earlier, at the pivotal battle of Yarmuk (636), where the Muslims crushed the Byzantines, leading to the conquest of Syria, one Muslim complained to the caliph, saying, “The dog of the Romans [Emperor Heraclius] has greatly frustrated us with the ubiquitous presence of the cross!”
The Siege of Byzantium In 717–18, Western civilization was hanging by a thread.
Seige of Constantinople, from the Constantine Manasses Chronicle.
More important, Constantinople — from a theological perspective — simply had to fall. From the start, Islam and jihad were inextricably linked. The jihad, or “holy war,” which took over Arabia and Persia, followed by Syria, Egypt, and all of North Africa — all formerly Byzantine territory — was considered a religious obligation, or, as later codified in sharia law, a fard kifaya: a communal obligation on the body of believers, to be adhered to and fulfilled no less than the Five Pillars of Islam.
As the famous 14th-century Muslim historian Ibn Khaldun put it: “In the Muslim community, the jihad is a religious duty, because of the universalism of the Muslim mission and the obligation to convert everybody to Islam either by persuasion or by force. . . . Islam is under obligation to gain power over other nations.”