There was a great deal of intertribal warfare occurring between scattered groups of Arabic peoples in Muhammad's day. What he did was no different than what was generally going on...except for this: he was better organized than the rest. His people were extremely well organized, bolstered by the new faith, and they succeeded in a series of military campaigns that united all those warring tribes under a single faith and gave the Arabs true nationhood for the first time. That's quite an accomplishment.

Everybody spread religion at the point of the sword in ancient times, mostly because religion was inextricably tied in with all the other political imperatives that usually lie behind wars: land, trade rights, waterways,resources of all kinds, political dominion, money, control, ambition, powerful families...the usual stuff.