To seize the Byzantine throne, Emperor AlexiosIII had blinded and imprisoned his elder brother, Isaac II Angelos (). Isaac's son, Alexios, fled from Constantinople to Germany to seek his Catholic relatives' assistance. He concluded an agreement with the leaders of the Fourth Crusade, promising a large sum, 800,000 "hyperpyra", for their support. The crusaders reached Constantinople and captured Pera on the opposite coast of the Golden Horn on 6July 1203. Theodore conducted raids against the invaders, but they laid siege to the Byzantine capital. The walls of Constantinople were vulnerable, and AlexiosIII fled in panic to Thrace during the night of 17–18 July. He drained the treasury and took the imperial insignia with him.