 A Venetian fleet forced Vatatzes ' naval forces to withdraw , but after the <unk> departed for home the Greeks and <unk> besieged Constantinople again in November 1235 . John sent letters to European monarchs and the pope , pleading for assistance . Since the survival of the Latin Empire was in jeopardy , Pope Gregory urged the crusaders to defend Constantinople instead of the Holy Land . A combined naval force from Venice , Genoa , <unk> and Geoffrey II of Achaea broke through the blockade . Asen soon abandoned his alliance with Vatatzes , who was forced to lift the siege in <unk> . 
