Islam started to spread when Prophet Muhammad received his first revelation at forty years of age when Angel Gabriel gave him the message from god. Prophet Muhammad first started preaching about Islam to his wife and closets friends in secret for three years. Later he started preaching openly in Mecca ten years and in Medina for ten years. Islam only really started spreading when Prophet Muhammad died. It was the four caliphs that were really determined to spread Islam around the world.
The military also helped to spread Islam around the world because when the Muslims fought in wars the had Rules of Conduct that they had to follow like how they cannot kill old people, girls and women. Also they were not allowed to poison water, kill animals, chop down trees and burn houses of god like temples. Those people who surrender against Islam became hostages and their families had to pay to set them free. If their families didn’t have money to pay then they would have to negotiate with their skills for example, the hostage is handy with medicine then he would have to help the hospital to heal the wounded for a certain number of years maybe five or six. Once they finish the five or six years then they are free to go. Then once the hostage is free some of them convert to Islam because the Muslims were always kind to them, giving them food and water.
Trading helped spread Islam because in order to trade goods and things you have to travel to different places. When the traders reach their destination then they would preach to the traders about Islam and the way of life because Islam is not just a religion but it is in fact a way of life. So the traders would tell them the Islamic way of living and the five pillars giving Zakat and the Hajj and everything. Some of the traders like the sound of the one god an their way of life so they convert to Islam and then they do the same to people who come to trade with Mecca.
Whenever the Muslims...