On 18 July 1203 the Crusaders launched an assault on the city, and Alexios III immediately fled into Thrace. The next morning, the Crusaders were surprised to find that the citizens had released Isaac II from prison and proclaimed him emperor, despite the fact that he had been blinded to make him ineligible to rule. The Crusaders forced Isaac II to proclaim his son Alexios IV co-emperor on 1 August, effectively ending the siege.