Hilaire Belloc asks, What would have happened if the First Crusade had taken back Syria from the men of the desert? in his book, The Battleground. At the end of Chapter 14, he answers:
...With the French chivalry permanently established in Syria, Islam would have been cut in two. Its eastern half might have survived, its western would have been doomed. The Mediterranean and its islands, which had fallen into the power of Islam, would have been Roman and Christian again; there would have been done in a fruitful Christian time what is now being attempted in our own sterile time of Apostasy. Roman land would have been recovered in its entirety and Christendom would have become Christendom again.
For Syria is the keystone. To hold Syria permanently, with sufficient recruitment and armament, is to cut the bridge between Asia, including the men of the desert, and North Africa. Syria strongly held makes the enemy hold over Egypt impossible, for Syria strongly held is the holding also of the neck between North Africa and the Levant. Syria strongly held cuts all advance from Asia towards the Bosphorus, for it flanks the highlands of Anatolia. Syria strongly held is the recovery of the Roman East. Had the Roman West been able to conquer Syria and hold it strongly now after this first enthusiasm of the Crusades, Islam would have been thrown back to where those other enemies had been before the defeat of the Byzantines on the Yarmuk. But that splendid effort-the last effort was to fail.
Oh, and check out this pro-Islam clip of a movie on the Battle of Yarmuk.