Why were the arabs able to conquer byzantium

Arab–Byzantine wars were a series of wars between the mostly Arab Muslims and the East Roman or Byzantine Empire between the 7th and 11th centuries AD, started during the initial Muslim conquests under the expansionist Rashidun and Umayyad caliphs in the 7th century and continued by their successors until the mid-11th century.

Afederal parliamentary republic is  unlimited.