Komnenous Doukas likewise had himself crowned emperor, establishing the Empire of Thessalonica, and came close to recapturing Constantinople before being defeated by Bulgaria at the Battle of Klokotnitsa in 1230.

After Klokotnitsa, Ivan Asen II of Bulgaria aspired to himself restore the Byzantine Empire, variously allying with and opposing the Nicene emperor John Doukas Vatatzes.

Although opposed by Bulgaria, the Thessalonian successor states, and the remaining Latin lords of Greece, the Nicenes under Michael Palaiologos ultimately captured Constantinople in 1261, restoring the Byzantine Empire.

[4] Although Byzantine territory gradually declined over the centuries, their claim to rule continued to be acknowledged by temporal and religious authorities in Western Europe, even if formal imperial control could not be restored.

[17] Isaac imprisoned several Latin citizens in Constantinople[18] and concluded a secret alliance with the Muslim ruler Saladin, promising to delay and destroy Barbarossa's army in return for concessions in the Holy Land.

As Isaac II laid dying of natural causes, the populace elected a new (unwilling) emperor in Hagia Sophia on 27 January 1204, Nicholas Kanabos.

[28] As a result of numerous great fires started during the Crusaders' stay in the city, about a sixth of Constantinople was a burned ruin and the rest was depopulated and stripped of anything valuable.

Following the sack of the city, the rest of the Byzantine Empire had unraveled into a patchwork of bewildered imperial governors, foreign invaders, rebels, and pretenders.

After the intervention of both other Crusaders and the Venetians, Boniface agreed to leave Baldwin's imperial domains and sell Crete to Venice, in return being allowed to go on and occupy Thessalonica on his own.

[47] Kaloyan, tsar of the Second Bulgarian Empire, had in reaction to the Latin successes approached Pope Innocent III, offering to accept the Catholic Church in return for a papal coronation.

[44] In the spring of 1206, Kaloyan conducted a brutal raid of Thrace,[50] styling himself as "the Roman-slayer" in imitation of the Byzantine emperor Basil II (r. 976–1025, who had been called "the Bulgar-slayer").

Michael of Epirus married his daughter to Eustace and made a formal submission to Henry in order to avoid invasion from the resurgent Latin Empire.

A Seljuk invasion of the Meander valley was defeated, Lesbos, Chios, and Samos were reconquered from the Latins, and Vatatzes aided a revolt by the natives of Crete against the Venetians.

Vatatzes also tried to negotiate with Pope Gregory IX, offering to work on reuniting the Catholic and Orthodox churches in return for the surrender of the Latin Empire.

Since Theodore could no longer officially rule himself, he had his younger son John Komnenos Doukas crowned as emperor in Thessalonica while he himself held the real power form the nearby town of Vodena.

[88] An outbreak of the plague in Bulgaria, which killed both high church officials and several of Asen's immediate family members was interpreted by the Bulgarian emperor as divine wrath for his treachery against the Nicenes.

[88] Later in the same year, Baldwin II returned to Constantinople, backed by an army of French soldiers he had raised through selling a Byzantine relic purported to be the crown of thorns to Louis IX of France.

Thessalonica and Epirus were allowed to remain independent, though Vatatzes annexed some Thracian territory and made John give up the title of emperor in favor of despot, nominally under Nicene suzerainty.

[94][96] Wishing to stop the Mongols from penetrating into Anatolia, Vatatzes concluded an alliance with Sultan Kaykhusraw II of Rum in 1243, promising to send him a detachment of 400 soldiers.

[99][100] In late 1251, Theodore Komnenos Doukas launched his last attempt at restoring his empire, conducting a joint campaign against the Nicene holdings in the Balkans with his nephew, Michael II of Epirus.

With the war lost, Michael agreed to accept Nicene suzerainty again, give his son Nikephoros as a hostage, and to cede Prilep, Kastoria, Ohrid, Vodena, as well as the hinterland of Dyrrachium to Vatatzes.

[104] A new Epirote attempt at recapturing Thessalonica fell through when Epirus was attacked by Manfred, regent of the Kingdom of Sicily, who captured most of Albania as well as the island of Corfu.

The alliance soon began to fracture due to internal disputes and Michael of Epirus's illegitimate son John Doukas, who ruled Thessaly, deserted to Palaiologos.

Although the Nicenes captured Selymbria and attacked Galata, the city resisted and the Palaiologoi were forced to sign a year's truce with Emperor Baldwin II.

He signed a treaty with Genoa, the main trading competitor of Venice, granting the Genoese large commercial concessions in Nicene ports in return for help in capturing Constantinople.

[108] As Strategopoulos closed in on Constantinople he learnt from the local Byzantines that both the Latin army and the Venetian fleet were away from the city, conducting a surprise attack on the Nicene island of Daphnusia.

[107] Three weeks after the reconquest, Michael Palaiologos made a triumphant entry into Constantinople[107] and on 15 August had himself crowned emperor in proper Byzantine ceremony in Hagia Sophia.

In the early 1270s, Charles occupied almost all of Albania and managed to gain the allegiance of John Doukas of Thessaly, as well as Konstantin Tih, who had sired a son with Maria without receiving the promised territories from Michael.

Although a diplomatic triumph, the church union was unpopular in much of Byzantium and was rejected by almost all Orthodox Christians outside the empire, including the rulers of Trebizond, Epirus, and Thessaly.

The new pope, Martin IV, declared that Michael had failed to achieve a true union of the churches, allowing Charles to again plan a crusade.