What empire blocked abbasid expansion to the northwest?

The Byzantine Empire blocked Abbasid expansion to the northwest. The Abbasid Caliphate, based in Baghdad, sought to expand its territory in various directions, including towards the northwest. However, the Byzantine Empire, with its strong military and strategic position, effectively prevented the Abbasids from expanding further in that direction. This geopolitical barrier played a significant role in shaping the boundaries and influence of both empires during that time period.