Why were Byzantines and Persians were surprised by by the strength of the Arab Muslims?

Byzantines and Persians were surprised by the strength of the Arab Muslims due to the latter's rapid military successes and the unity they exhibited under Islam, which was a stark contrast to the fragmented and often weakened state of the Byzantine and Persian empires. The Arab armies were highly motivated by religious fervor and a desire for expansion, which allowed them to execute swift and coordinated campaigns. Additionally, the internal strife and exhaustion from previous conflicts within both empires left them ill-prepared for the unexpected and aggressive military tactics of the Arab forces. This combination of factors led to significant Arab victories, altering the balance of power in the region.