Before we consider the borderlands between Byzantium and its Muslim neighbours, it will be necessary to start with a well-known commonplace: when speaking of borders today, we think of definite lines that are largely impenetrable. Well-known modern examples are the Berlin Wall, or the ‘Trump Wall’ planned between USA and Mexico. Maps in historical atlases invoke a similar impression. In fact, however, in earlier times this was extremely rarely the case, as with the Roman Limes in Germany, Hadrian's Wall between England and Scotland, or the Chinese Great Wall.

These, however, were great exceptions. As a rule, no defined borders existed, except perhaps at a river or a coastline, and even there, they were not impassable. Just one example: although Byzantium was able successfully to defend the border at the Danube in the sixth and early seventh century, it could nevertheless not prevent this line being breached repeatedly by Avars, in particular Slavs. Thus, control of the hinterland was lost almost completely, leading to the very loss of the borderline itself some decades later. The more usual case in the pre-modern world, however, was that borders between different powers were usually blurred: regions that they would control more or less, usually less.

But it is certainly important to differentiate between times of peace and those of warfare: in peace, a closely guarded borderline was unnecessary. It sufficed to establish specific checkpoints, be it directly at the border or in the hinterland, where foreigners, merchants, pilgrims, or other travellers entered or exited imperial territory, or where they were otherwise required to report to authorities. Well-known examples include, in the Balkans, Thessaloniki, or later Belgrade in the Middle Byzantine period; Laodicea and Antioch in Northern Syria; and Trebizond on the Black Sea coast. To Constantinople, access, as a rule, was generally restricted.

Equally important is another factor for the fate of the borderland: controlling border regions became important and at once problematic when confronting powers pursuing inherently aggressive politics. This was almost always the case at the eastern border during the Umayyad and the Abbasid dynasties, and later after the arrival of the Seljuqs and the Turcoman nomads. But what could Byzantium do to ward off these threats?