Following the siege of Constantinople and the dissolution of the Byzantine Empire by the Fourth Crusade in April 1204, Thessaly passed to Boniface of Montferrat's Kingdom of Thessalonica in the wider context of the "Frankokratia". In 1212, Michael I Komnenos Doukas, ruler of Epirus, led his troops into Thessaly. Larissa and much of central Thessaly came under Epirote rule, thereby separating Thessalonica from the Crusader principalities in southern Greece. Michael's work was completed by his half-brother and successor, Theodore Komnenos Doukas, who by 1220 completed the recovery of the entire region.