For access to more resources, spreading ideologies (white man’s burden)
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.
The European settlers treated the native Americans like slaves. they invaded their space, set up gold mines and didnt show any respect for them or their beliefs at all. lots of native Americans were killed by the settlers, and there were many wars over control of the land. i hope this helps :)
