Question

Fan and medal

What challenges did the Byzantine Empire face from foreign forces?
Slavs and Avars–
Avars and Sassanids–
Arabs–
Bulgars–

Answer 1

Most empires face the challenge of REMAINING EMPIRES, don't they ?
Eg, the "British Empire" after the Fall of Singapore in ww2, and the eventual renaming of the territories as the "Commonwealth" rather than the "Empire".
In the case of the post possibly a number of wars to prevent invaders invading and to prevent "colonies" rebelling maybe.