Why did the Byzantine emperor Isaac II Angelus try to delay Frederick I’s Crusader army?

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Medieval manuscript miniature showing Frederick I as a crowned crusader holding an orb and scepter, with Henry of Schaftlarn kneeling and presenting a book to him. — Frederick I Frederick I as a Crusader with (right) Henry of Schäftlarn dedicating to him a copy of the *History of the First Crusade* by Robert of St. Remy; miniature from a manuscript in the Vatican Library.

Byzantine emperor Isaac II Angelus attempted to impede the progress of Frederick I’s Crusader army through Greece because he had entered into a secret treaty with Saladin, the sultan of Egypt, to protect his own interests.12

This obstructionist policy led to conflict, during which Frederick captured the city of Adrianople. Frederick only returned the city to Byzantine control after Isaac II agreed to transport the German forces across the Hellespont into Asia Minor, a commitment formalized in the Treaty of Adrianople in February 1190.12