Where did Islam spread? Why did Islam spread?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Islam is the only true religion; Allah does not accept any this religion.  Allah Says (interpretation of meaning): {And whoever seeks a religion other than Islâm, it will never be accepted of him, and in the Hereafter he will be one of the losers.}[3:85].

Islam is a light, Allah Says (interpretation of meaning) about it: {….Indeed, there has come to you from Allâh a light (Prophet Muhammad SAW) and a plain Book (this Qur'ân).}[5:15].  Allah also Says (interpretation of meaning): {They intend to put out the Light of Allâh (i.e. the religion of Islâm, this Qur'ân, and Prophet Muhammad SAW) with their mouths. But Allâh will complete His Light even though the disbelievers hate (it).}[61:8].

The only reason for the spread of this religion is the clear and cogent proofs of the truthfulness of this religion.

Allah knows best.