Unfortunately, there have been periods when the Middle east has been basically unified and during those periods it was a great danger to Western Civilization. It already is again and it is not even unified.
Perhaps I just don't know enough about Islamic and Arabic civilization, but I can't identify that period when Islam was truly united, except under the early "rightly guided" caliphs.
The rest of their long history has been filled with Fatimids, Buyids, Ummayads, Abbasids, Mamelukes, and whatever you like flavor of powers and potentates. They were only dangerous at Tours and after Constantinople fell -- and that was the Turks, not the Arabs; the Turks would have been a threat whether they'd converted to Islam or not, like their distant cousins the Mongols, who came as far as Liegnitz in the north (1241) and Ain Jalut (Well of Goliath, 1260) in the south.