The Crusades Could Have been Stopped at the Beginning

بِسۡمِ ٱللهِ ٱلرَّحۡمَـٰنِ ٱلرَّحِيمِ

In the late 11^th century, much of Syria was divided among petty Seljuk princes who contended with each other.  They were so divided that the Crusader armies could march right through hundreds of miles of Muslim territory to reach Jerusalem.  They not only did not hinder the Crusaders, but they also helped them. 

The Crusades could have ended at the gates of Antioch, the first major city in the Levant they encountered.  The emir of the city was Yaghi Siyan, and he was so hated by his rivals, the emirs of Aleppo, Mosul, and Damascus, that they preferred to see the city fall to the Crusaders, rather than help him.  And that is how it began, and that is also why the Crusaders were in Muslim lands for over a hundred years. 

The Muslims never learned their lesson, of course, and that was the reason for the Fall of al-Andalus, for their defeat in Central Asia, and for the eventual colonialisation of all Muslim lands.  And that disease afflicts us to this day.