Nicaea, empire of, 1204-61. In 1204 the armies of the Fourth Crusade set up the Latin Empire of Constantinople, but the Crusaders' influence did not extend over the entire Byzantine Empire. Several Greek successor states, chief among them the empire of Nicaea, sprang up (see also Epirus, despotate of; Trebizond, empire of). The empire of Nicaea preserved the continuity of emperors, patriarchs, and institutions of Byzantium. Founded by Theodore I (Theodore Lascaris) in NW Asia Minor, with Nicaea as its capital, it played the decisive part in reuniting the Byzantine Empire. Theodore I and his successors of the Lascaris family expanded their domains, defeated their neighbors to the south, the Seljuk Turks, and in alliance with Ivan II of Bulgaria weakened their chief rivals, the despots of Epirus. They successfully warred against the Latins, and when the Mongol invasions weakened the Turks of Iconium, Nicaea became supreme in Asia Minor. Michael VIII (Michael Palaeologus), who usurped the throne of Nicaea in 1259, captured Constantinople from the Latins and restored (1261) the Byzantine Empire.

Nicaea, Second Council of, 787, 7th ecumenical council, convened by Byzantine Empress Irene. Called to refute iconoclasm, the council declared that images ought to be venerated (but not worshiped) and ordered them restored in churches. Practically the only Western delegates were the papal legates, but popes have confirmed the conciliar canons. It is the last council accepted by both the Roman Catholic Church and the Orthodox Eastern Church as ecumenical.

Nicaea, First Council of, 325, 1st ecumenical council, convened by Roman Emperor Constantine the Great to solve the problems raised by Arianism. It has been said that 318 persons attended, but a more likely number is 225, including every Eastern bishop of importance, four Western bishops (among them Hosius of Córdoba, president of the council), and two papal legates. The chief figures at the council were Arius and his opponent, Athanasius. The council adopted, as a test of faith, a formula that seems to have been based on a simple baptismal creed presented possibly by Eusebius of Caesarea; this was not, however, the creed generally circulated today as the Nicene Creed (see creed). The formula included the Greek word homoousion [consubstantial], which was used concerning the Son and the Father. The word, suggested probably by Hosius, became the touchstone of orthodoxy and the bugbear of Arianism, for it established the divinity and the equality of the Son to the Father. The creed was accepted by all the bishops except two, who were banished along with Arius to Illyricum. The council ruled on other questions as well, attempting to standardize the date of Easter and granting patriarchal authority to the bishop of Alexandria. The First Council of Nicaea was significant as the model and the original of great councils. The test it adopted provided a universal statement of faith in place of the earlier and varying baptismal formulas.

Nicaea, city of Bithnyia, N Asia Minor, built in the 4th cent. B.C. by Antigonus I as Antigonia and renamed Nicaea by Lysimachus for his wife. It flourished under the Romans and was the scene of the ecumenical council called in A.D. 325 by Constantine I. Another council held in 787 sanctioned the devotional use of images. The city, captured by the Turks in 1078 and by the Crusaders in 1097, passed finally to the Turks in 1330. It is sometimes called Nice. The modern Iznik, Turkey, is on the site.

Independent principality (1204–61) of the fragmented Byzantine Empire. Founded in 1204 by Theodore I Lascaris, it was the political and cultural centre from which a restored Byzantium arose in the mid-13th century under Michael VIII Palaeologus. It extended from the Black Sea coast east of the Sangarius River southwest across western Anatolia to Miletus and the Menderes (Maeander) River. It became a centre of Greek education, especially under Theodore II Lascaris, who founded an imperial school. It declined after 1261, when Michael VIII regained the Byzantine capital of Constantinople.

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