In 1204, Byzantine emperor Alexios V Ducas Murtzouphlos fled Constantinople after crusaders invaded the city. Soon after, Theodore I Lascaris, the son-in-law of Emperor Alexios III Angelos, was proclaimed emperor but he too, realizing the situation in Constantinople was hopeless, fled to the city of Nicaea (today İznik) in Bithynia.
The Latin Empire, established by the Crusaders in Constantinople, had poor control over former Byzantine territory, and Greek successor states of the Byzantine Empire sprang up in Epirus, Trebizond, and Nicaea. Trebizond had broken away as an independent state a few weeks before the fall of Constantinople. Nicaea, however, was the closest to the Latin Empire and was in the best position to attempt to re-establish the Byzantine Empire.
Theodore Lascaris was not immediately successful, as Henry of Flanders defeated him at Poimanenon and Prusa (now Bursa) in 1204, but Theodore was able to capture much of northwestern Anatolia after the defeat of Latin Emperor Baldwin I in the Battle of Adrianople, Henry was recalled to Europe to defend against invasions from Kaloyan of Bulgaria. Theodore also defeated an army from Trebizond, as well as other minor rivals, leaving him in charge of the most powerful of the successor states. In 1206, Theodore proclaimed himself emperor at Nicaea.
Numerous truces and alliances were formed and broken over the next few years, as the Byzantine successor states, the Latin Empire, the Bulgarians, and the Seljuks of Iconium (whose territory also bordered Nicaea) fought each other. In 1211, at Antioch on the Meander, Theodore defeated a major invasion by the Seljuks, who were backing a bid by Alexios III Angelos to return to power. The losses suffered at Antioch, however, led to a defeat at the hands of the Latin Empire at the Rhyndacus River and the loss of most of Mysia and the Marmara Sea coast in the subsequent Treaty of Nymphaeum. The Nicaeans were compensated for this territorial loss when, in 1212, the death of David Komnenos allowed their annexation of his lands in Paphlagonia.
Theodore consolidated his claim to the imperial throne by naming a new Patriarch of Constantinople in Nicaea. In 1219, he married the daughter of Latin Empress Yolanda of Flanders, but he died in 1222 and was succeeded by his son-in-law John III Ducas Vatatzes.
The accession of Vatatzes was initially challenged by the Laskarids, with the sebastokratores Isaac and Alexios, brothers of Theodore I, seeking the aid of the Latin Empire. Vatatzes prevailed over their combined forces, however, in the Battle of Poimanenon, securing his throne and regaining almost all of the Asian territories held by the Latin Empire in the process.
In 1224, the Latin Kingdom of Thessalonica was captured by the Despot of Epirus Theodore Komnenos Doukas, who crowned himself emperor in rivalry to Vatatzes and established the Empire of Thessalonica. It proved short-lived, as it came under Bulgarian control after the Battle of Klokotnitsa in 1230. With Trebizond lacking any real power, Nicaea was the only Byzantine state left, and John III expanded his territory across the Aegean Sea. In 1235, he allied with Ivan Asen II of Bulgaria, allowing him to extend his influence over Thessalonica and Epirus.
In 1242, the Mongols invaded Seljuk territory to the east of Nicaea, and although John III was worried they might attack him next, they ended up eliminating the Seljuk threat to Nicaea. In 1245, John allied with the Holy Roman Empire by marrying Constance II of Hohenstaufen, daughter of Frederick II. In 1246, John attacked Bulgaria and recovered most of Thrace and Macedonia, and proceeded to incorporate Thessalonica into his realm. By 1248, John had defeated the Bulgarians and surrounded the Latin Empire. He continued to take land from the Latins until his death in 1254.
Theodore II Lascaris, John III's son, faced invasions from the Bulgarians in Thrace, but successfully defended the territory. A conflict between Nicaea and Epirus broke out in 1257. Epirus allied with Manfred of Sicily when Theodore II died in 1258. John IV Lascaris succeeded him, but as he was still a child he was under the regency of the general Michael Palaeologus. Michael proclaimed himself co-emperor (as Michael VIII) in 1259, and soon defeated a combined invasion by Manfred, the Despot of Epirus, and the Latin Prince of Achaea at the Battle of Pelagonia.
Recapture of Constantinople
Coin issued by Michael VIII Palaeologus
to celebrate the liberation of Constantinople from the Latin army, and the restoration of the Roman/Byzantine Empire.
In 1260, Michael began the assault on Constantinople itself, which his predecessors had been unable to do. He allied with Genoa, and his general Alexios Strategopoulos spent months observing Constantinople in order to plan his attack. In July 1261, as most of the Latin army was fighting elsewhere, Alexius was able to convince the guards to open the gates of the city. Once inside he burned the Venetian quarter (as Venice was an enemy of Genoa, and had been largely responsible for the capture of the city in 1204).
Michael was recognized as emperor a few weeks later, restoring the Byzantine Empire. Achaea was soon recaptured, but Trebizond and Epirus remained independent Byzantine Greek states. The restored empire also faced a new threat from the Ottomans, when they arose to replace the Seljuks.