There can be little doubt, however, that the [Islamic) conquests were made easier by the exhaustion of the Byzantine and Sasanian Empires due to prolonged warfare, the confusion that reigned in the Sasanian ruling house, the disruption caused by recent enemy occupation in Syria and Iraq, the
destruction wrought by immense floods in
southern Iraq, the disaffection of many of the subjects of the two empires for religious or other reasons, the convenience of inner lines of communication
that the Muslims enjoyed, and the like.
Fred Donner, The Early Islamic Conquests, Princeton University Press,
1981
According to the information in the excerpt, one reason Islam was able to spread was because –
O nearby revolts were taking place
O Muslim soldiers were highly trained
O neighboring empires were weak
O geography made invasion easy