Islam, as formed by Mohammed ... was founded in a region where polytheism was rampant. The moon god, "ALLAH", was selected by Mohammed (most Muslim countries have a crescent moon and star on their flag). Mohammed (an illiterate but smart businessman- warrior - leader - narrated the Koran to a scribe. The early Koran has contradictions with the later writings. (This helped fuel the book by Salmud Rushdie - the "Satanic Verses" ... where the premise is that the scribe for Mohammed actually writes different words than what the "Prophet" Mohammed directs, and given Mohammed's inability to read, the Prophet doesn't catch the mistakes. And the early Koran is more peaceful, while the later Koran has more hate for non-believers. (This is the reverse of the Bible with the Old Testament and New Testament . . . where the New Testament is more on forgiveness, loving your enemy, and persuading people to belive, rather than forcing them to believe at the point of a sword. But even the OT was more peaceful to other people and religions (outside of the Promised Land).)
Of course, the Old Testament and New Testament carry the concept that a Prophet of God speaks the Word of God, and must be accurate and without error, and that God has preserved His Word through the centuries. It would seem that the Koran is filled with contradictions and errors, and would fail the test that the Bible is held to. (Let us not get into the Old Universe/Young Universe controvesy). There are some excellent books that do a critical analysis of the errors of the Koran (also written as "Qur'an" ). Might I suggest: "The Qur'an and the Bible, in the light of history and science", by Dr. William Campbell.
There is lot of info on Mohammed ... but the bottom line is he was more friendly to Jews and Christians in the early times, and then became extremely destructive and intolerant to them in later years. For those who criticize the Crusades as being cruel ... they forget that the Muslims invaded the Christian/Jewish Holy Land that had been peaceful, and forced conversions (accept or die), and set in motion all the ensuing horror that happened during the several Crusades.
Bottom line: Allah is not the same God as the Christian and Jewish God. Islam rejects much of the Old Testament and New Testament; they consider themselves to be Abraham's offspring through Ismael, the son of Hagar (Sarah's handmaid) - and therefore they consider much of the Old Testament, which is about the offspring of Abraham's son Isaac, irrelevant. Islam rejects the idea that Jesus is the Son of God, and they reject many of the miracles attributed to Jesus.
Mike