Why were the Arabs able to conquer much of Persia and parts of byzantium

It was easy for the Muslim army of the Arabs to conquer much of persia and parts of the Byzantium empire because bothe the persian and the Byzantium army were weak due to years of fighting among themselves. The Arabs did not have to face any tough resistance from either the Persian or the Byzantium army. During the first century, the byzantium army did not like fighting open battles and they mostly were in the defensive. Although during 740 the Byzantium army came out for open fighting with the arabs, but still they were battered with heavy artilerry by the Arab army.