Conquered people liked Islam's values and the way they would be treated under it so they decided to convert willingly rather than being forced to leading to more loyal citizens.
and
Muslims were unified
The splendors of Baghdad
The treatment of conquered people
. Empire of Caliphs
The First Caliph
Harshness of Persia and the Byzantines
Weakness of Persia and the Byzantines
Muslims in Europe
I hope this helps!