In 1099 the crusaders captured antioch but found they were surrounded by a newly are rived Muslim army.

This army was led by Kerbogha, who was the Atabeg of Mosul.

Explanation:

Kerbogha, after obtaining approval from the Caliph in Baghdad, mustered a large army and went to war. However, he made the crucial mistake of not going immediately to Antioch but to Edessa. In Edessa, Baldwin Bouillon established his principality after separating from the Crusade. Kerbogha therefore wanted to conquer Edessa first and then to Antioch. Edessa was a tough nut after all, and after three lost weeks trying to conquer Kerbogha, he gave up and headed for Antioch. Because of this, he was late and came when the city had already fallen into the hands of the cruisers.