As ruler of Thessalonica, Theodore quickly declared himself emperor, challenging the Nicaean emperor John III Doukas Vatatzes's claims to the Byzantine imperial throne. In 1225, he advanced to the outskirts of Constantinople, but his final attack against the seat of the much-reduced Latin Empire was delayed until 1230. In that year, Theodore amassed an army to besiege Constantinople, but then diverted it against Bulgaria, an ambivalent ally which threatened his northern flank. Theodore was defeated and captured at the Battle of Klokotnitsa, and spent the next seven years in captivity. In the meantime, he was succeeded by his brother Manuel. Manuel quickly lost Thrace, most of Macedonia, and Albania to the Bulgarian Emperor John Asen II. Thessalonica itself became a Bulgarian vassal, while in Epirus proper power was seized by Michael II, returning from exile.