Why did Islam spread?

Islam spread rapidly after the death of the prophet Muhammed. During the Rashidun caliphate, the rule of the first four caliphs or successors to Muhammed, the new religion spread far and wide through military conquest, trade, pilgrimage, and missionaries.
Many of the communities that came into contact with Islam for the first time welcomed it as a stabilizing factor. For instance, the neighboring Byzantine and Sasanian Empires had been significantly weakened by years of war with the Turks. This weakness gave Arab Muslims an opening, an opportunity to provide the subject peoples of these empires with peace and stable government.
As well as achieving a string of military victories, the Rashidun caliphate was able to unite the disparate Arab tribes into a cohesive political force. Though this alliance proved only temporary, it lasted long enough to allow Islam to spread throughout the Arabian peninsula, thus providing a strategic launchpad for incursions into neighboring territories.