Why did Frederick I capture Adrianople during the Third Crusade?

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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.

Frederick I captured the city of Adrianople during the Third Crusade as a direct response to the actions of the Byzantine emperor, Isaac II Angelus.

While Frederick’s army was passing through Byzantine territory, Isaac II attempted to impede their progress, motivated by a secret treaty he had made with Saladin. By seizing Adrianople, Frederick pressured the Byzantine leadership to cooperate. He ultimately returned the city to Byzantine control only after Isaac II agreed to transport the German forces across the Hellespont into Turkey, allowing the Crusaders to continue their journey toward the Levant.12