This thread will chronicle a world in which Mohammed (‘peace be upon him’) is defeated at the Battle of the Trench in 627. Medina is subjugated to the power of Mecca, and the Mohammedans never regain their former momentum becoming yet another political-religious group within the convulsing Arabian landscape of the early 7th century.
However, this does not mean that the Arabs don't experience some kind of 'Volkswanderung'. This timeline will not be simply a characterization of Turtledove's 'Agent of Byzantium' series (where St. Mahomet was the biggest Christian converter since Paul). The 6th century will not continue without end, and the 7th century will see lots of interesting new and dynamic changes for the Mediterranean East as it changes from late antiquity to early medieval.
In a no-Islam timeline, the Arab impact will still be dramatic and significant. The comparison could made with the Vikings. The kind of broad long-lasting impact on northern European civilization the Vikings had could be mirrored in the Near East by the Arabs, providing linguistic and cultural input as well as becoming the ruling class of many places. But just as the Vikings were in some sense absorbed by the Christian civilization of Europe, so the Arabs of this ATL (lacking the sense of unity that Islam provided) will be absorbed linguistically, culturally and religiously.
Anyway, please enjoy the following timeline, and remember all comments, criticisms, suggestions, condemnations, analyses and observations welcome.