This is a very broad question. Are we talking about the very advent of Islam in Arabia? Are we talking about its spread to the Levant, North Africa, and Spain? What about further abroad in Central Asia or Indonesia? Islam spread over the course of hundreds of years on multiple continents, so it would be impossible to give any sort of categoric answer to this question. We face the same question when it comes to tolerance. What occurred under the Umayyads in Spain was vastly different than the Safavids in Iran or the Mughals in India.

I did write up something a while back that talks about To what extent was the spread of Islam bloodless? and focuses on early Islamic expansions in Arabia itself. In the same thread, /u/sunagainstgold links to some other past answers that may be of interest.

In a question on Were the early Muslims colonial occupiers? (particularly the case of the Rashidun conquest of Egypt) I talk about the Arabization and Islamization of Egypt, for a more concrete look at one country.

I hope this is good to get you started and can start to answer your (extremely broad) question.