Ivan Asen and Vatatzes made an alliance against the Latin Empire. The Bulgarian troops conquered the territories to the west of the Maritsa, while the Nicean army seized the lands to the east of the river. They laid siege to Constantinople, but John of Brienne and the Venetian fleet forced them to lift the siege before the end of 1235. Early next year, they again attacked Constantinople, but the second siege ended in a new failure.