Another answer from our community:
Spread of Islam has positive effects and no negative effect. Islam religion accepts other religions but other religions do not. Muslims believe in all God prophets including Jesus and Moses and accordingly they respect their followers. However, Christians and Jews don't believe in Muhammad, Muslims prophet. That is why you never hear in history about Muslims committing Massacres to non Muslims but you find in history many non Muslims committing massacres to Muslims. Compare when Muslims took control of Spain how they spread peace and civilization while when non Muslims gained control of the country what massacres they made against Muslims.
Islam is a religion of submission to God and of peace and love but media visualize it in opposite sense to serve their own political agendas.
It is true that overall, the spread of Islam was more benign and peaceful than that of Christianity, but this is not a reason to take a starry-eyed view of Islam.
Most non-Muslim scholars step gingerly around the issue of Muslim atrocities, but Ibn Warraq is clearly an apostate and so both well-informed and more willing to be forthright in his book, Why I am Not a Muslim. It is easy to criticise him because of his apostacy, but he does cite credible scholars and evidence when he discusses contentious issues. Some of his claims:
the battle of Karbala had a great effect on spread of shiite Islam and caused Muslims specially in understand there is two main version of Islam, Islam of Yazid and Islam of Hussain S.A.
Sunni Islam spread throughout the eastern Mediterranean region.
The religion of Islam was spread. (study island answer)
Islam spread to libya by other countries
A struggle or conflict to spread Islam is a Jihad
Islam never spread by war. Refer to question below.
Islam spread north at first and then spread east and west.
It was spread in Arabic.
The prophets of god spread Islam.
to what regions, and in what general order, had Islam spread by 750
to what regions, and in what general order, had Islam spread by 750
Islam never spread by war. Refer to question below.