The Empire of Nicaea was the Byzantine government in exile from 1204 to 1261, the strongest of the successor states, and the one that took Constantinople back.
When the Fourth Crusade took Constantinople in 1204, the Byzantine state did not so much fall as scatter. Three Greek successor states emerged — at Nicaea in western Anatolia, in Epirus in north-western Greece, and at Trebizond on the Black Sea — and each claimed to be the empire.
Nicaea won that argument. It held the richest surviving Byzantine territory, it secured the patriarchate in exile, and it was close enough to Constantinople to make its claim credible and far enough away to be defensible.
For fifty-seven years it functioned as a working Byzantine state in miniature, and in 1261 it recovered the capital and dissolved itself into the restored empire.
Theodore I Laskaris, a son-in-law of the last pre-conquest emperor, established himself at Nicaea in 1204 and spent a decade fighting the Latins, the Seljuks and rival Greek claimants simultaneously.
His decisive achievement was legitimacy rather than territory: in 1208 he had a patriarch elected at Nicaea, who crowned him emperor. That gave the Nicaean claim a canonical foundation the other successor states never matched.
He beat the Seljuk sultan at Antioch on the Maeander in 1211 — killing him in single combat, according to the Greek sources — and secured a frontier that held for fifty years.
His son-in-law John III Vatatzes, who ruled from 1221 to 1254, is the reason Nicaea won. He was an administrator before he was a general, and he made the state solvent.
He pursued deliberate economic self-sufficiency — encouraging agriculture, banning the import of Italian luxury goods, and funding the treasury from imperial estates. The chronicler records him buying his wife a crown from the profits of the imperial poultry farms, which is the kind of detail that gets repeated because it is true to the man.
On that base he took the Latin possessions in Asia, crossed into Europe, took Thessalonica from Epirus in 1246, and left his successors a state that surrounded Constantinople on three sides.
Theodore I Laskaris (1205–1221) founded the state and secured its legitimacy with a patriarch and a coronation.
John III Vatatzes (1221–1254) made it solvent and expanded it into Europe; he was later venerated as a saint in Asia Minor, which no other Byzantine emperor of the period managed. Theodore II Laskaris (1254–1258) was learned, ill and suspicious, and his short reign alienated the aristocracy.
John IV Laskaris (1258–1261) was seven at his accession and eleven when Michael VIII Palaiologos, his regent and then co-emperor, had him blinded. Michael VIII (1259–1282) took the state to Constantinople and ended it there.
The single most important thing Nicaea held was not territory but the patriarchate. When a patriarch was elected there in 1208 and crowned Theodore I, the Orthodox church outside Latin control had a head, and it was at Nicaea rather than at Arta or Trebizond.
That gave the state a claim no army could supply, and it made Nicaea the address for Orthodox Christians across the eastern Mediterranean — including in territory it did not govern. Negotiations with Rome over church union ran through Nicaea for the whole period, always inconclusively.
The court was also a serious intellectual centre. Nikephoros Blemmydes taught there, Theodore II Laskaris wrote philosophy as well as ruling, and the libraries and schools assembled in exile are part of what the restored empire carried back to Constantinople in 1261.
Pelagonia in 1259 destroyed the coalition that might have stopped Nicaea, and in July 1261 a Nicaean force walked into Constantinople through an undefended gate.
The empire of Nicaea then ceased to exist, not by conquest but by becoming what it had always claimed to be.
Whether that was a good outcome is a genuine historical argument and worth stating as one. Nicaea was compact, defensible, solvent and Anatolian; the restored empire was overextended, broke, and committed to defending a capital it could not fill. Some historians have argued the Laskarid state would have lasted longer than the Palaiologan one did. It is unprovable and it is not unreasonable.