The Empire of Nicaea was the largest of the three Byzantine Greek successor states founded by the aristocracy of the Byzantine Empire that fled after Constantinople was occupied by Western European and Venetian forces during the Fourth Crusade. Founded by the Laskaris family, it lasted from 1204 to 1261, when the Nicaeans restored the Byzantine Empire in Constantinople.
In 1204, Byzantine emperor Alexios V Ducas Murtzouphlos fled Constantinople after crusaders invaded the city. 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.
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.