The Epirote successes in Macedonia worried the Latins, as they opened the way to another attack on Thessalonica. The Kingdom of Thessalonica had been much weakened after the death of its founder, Boniface of Montferrat, in 1207, since which it had been ruled by a regency for his underage son Demetrius (). Interrupting a campaign against the Nicaeans in Asia Minor, Henry of Flanders hastened to Thessalonica. He took up contact with Boril and was preparing to march against Theodore when he suddenly died on 11 June 1216, probably of malaria, although poisoning by his second wife Maria of Bulgaria has also been suggested. The death of the warlike Henry, followed by that of Pope Innocent III, the instigator of the Fourth Crusade, a month later, was a major stroke of good fortune for Theodore as it removed two of his most eminent and capable opponents.