The Byzantine Empire
Emperor Henry
In 1211, both Nicaea and Bulgaria again took the offensive against Byzantium. Henry achieved some victories in the north, then crossed the Bosporus in October. Fighting with a very small force (only 260 knights) against much larger armies, Henry won a series of great victories. By the end of the year he had conquered much of the Ionian coast and had forced Lascaris to sign a treaty recognizing the conquests. Nicaea kept Pergamum and lands further south, but the coast north of there and along the Sea of Marmora was now Latin.
Bulgaria again fell into civil war in 1213, and Henry was adept at meddling, supporting one of the claimants. In 1214, Michael of Epirus was murdered. He was succeeded by his brother Theodore (1214-1230), who was an ally of Theodore Lascaris. Emperor Henry died June 11, 1216, at the age of forty. The barons chose Peter of Courtenay as the new emperor, and Conon of Béthune served as bailie until Peter should arrive from Europe. He went to Rome and was crowned there by the pope. With an army of 6,000 he crossed the Adriatic and briefly besieged Durazzo, which had been captured by Theodore of Epirus. Unable to take the city quickly, he raised the siege and marched overland for Constantinople, but was captured by Theodore along with his army. He was imprisoned in Epirus and died there in 1219, having never reached his own empire.
Peter had sent his wife Yolanda ahead by sea. She was pregnant at the time and gave birth in Constantinople to a son, who would become Baldwin II. With her husband in prison, she ruled as empress until her own death in 1219. While she lived, she married one of her daughters to Geoffrey II Villehardouin, and her other daughter to Theodore Lascaris, who had been at peace with Byzantium since his defeats in 1211. After Yolanda died, Conon of Béthune again served as bailie. The new emperor chosen was Robert of Courtenay.
Robert came east in 1220 and was crowned emperor March 25, 1221. Fighting had already begun, as Theodore Lascaris broke his treaty with Byzantium soon after Yolanda's death. Lascaris died in August 1222 and was succeeded by his daughter's husband, John Ducas Vatatzes (1222-1254).
Vatatzes proved to be a formidable enemy. He renewed the war in 1224 and by 1225 had driven the Byzantined almost completely out of Asia Minor. He also launched an invasion of Gallipoli.