In 623, shortly after establishing his Islamic community in Medina, Muhammad's Muslims began raiding caravans en route to Mecca. In retaliation, over the next 8 years, determined to quash Muhammad's power and the spread of Islamic teachings, the Meccans repeatedly attacked the Muslims of Medina - often with superior forces. Sometimes the Muslims lost battles, but they were never completely defeated. Jewish tribes accused of supporting enemies of Islam were slain or enslaved. In 624, the Battle of Badr saw 1,000 Meccans defeated by only 300 Muslims.
In 627, Meccans attacked Medina but failed and withdrew after one month. In 628, emboldened by their victory, Muslims conquered the Jewish community of Khaybar - an oasis 95 miles north of Medina - and slaughtered or expelled all of it's residents. Arab tribes quickly began sending delegates to submit to Muhammad and convert to Islam, rapidly increasing Muhammad's power.
Note: It should be recognized that religious and political conquest was accepted as the common method of social change in Muhammad's era and for thousands of years previously and for hundreds of years afterward.
In the same year that Muslims repelled the Meccans, 627, the Byzantine Empire, successors of the Holy Roman Empire, decidedly defeated the Persians in Ninevah, an ancient "exceeding great city" located at what is now Mosul, in northern Iraq.
A little over 450 years later, Pope Urban would declare "bellum sacrum" ("Holy War") against the Muslims, promising the remission of sins - and therefore assuring a place in heaven - to any western Christians who came to the aid of the Greeks because, the pope said, "Deus vult!" ("God wills it!").