Reconquest of Constantinople in the context of "Empire of Nicaea"

The sack of Constantinople occurred in April 1204 and marked the culmination of the Fourth Crusade. Crusaders sacked and destroyed most of Constantinople, the capital of the Byzantine Empire. After the capture of the city, the Latin Empire (known to the Byzantines as the Frankokratia, or the Latin occupation) was established and Baldwin IX of Flanders crowned emperor in Hagia Sophia.

After the city's sacking, most of the Byzantine Empire's territories were divided up among the Crusaders. Byzantine aristocrats also established a number of small independent splinter states—one of them being the Empire of Nicaea, which eventually recaptured Constantinople in 1261 and proclaimed the reinstatement of the Empire. However, the restored Empire never managed to reclaim all its former territory or attain its earlier economic strength, and it gradually succumbed to the rising Ottoman Empire over the following two centuries.

The Byzantine–Ottoman wars were a series of decisive conflicts between the Byzantine Greeks and Ottoman Turks and their allies that led to the final destruction of the Byzantine Empire and the rise of the Ottoman Empire. The Byzantines, already having been in a weak state even before the partitioning of their Empire following the 4th Crusade, failed to recover fully under the rule of the Palaiologos dynasty. Thus, the Byzantines faced increasingly disastrous defeats at the hands of the Ottomans. Ultimately, they lost Constantinople in 1453, formally ending the conflicts (however, several Byzantine Holdouts lasted until 1479).

Taking advantage of the situation, the Seljuk Sultanate of Rum began seizing territory in western Anatolia, until the Nicaean Empire was able to repulse the Seljuk Turks from the remaining territories still under Byzantine rule. Eventually Constantinople was re-taken from the Latin Empire in 1261 by the Nicaean Empire. The position of the Byzantine Empire in Europe remained uncertain due to the presence of the rivals in Epirus, Serbia and Bulgaria. This, combined with the declining power of the Sultanate of Rum (Byzantium's chief rival in Asia Minor) led to the removal of troops from Anatolia to maintain Byzantium's grip on Thrace.

Michael VIII Palaiologos or Palaeologus (Greek: Μιχαὴλ Δούκας Ἄνγελος Κομνηνὸς Παλαιολόγος, romanized: Mikhaḗl Doúkās Ángelos Komnēnós Palaiológos; 1224 – 11 December 1282) reigned as Byzantine emperor from 1261 until his death in 1282, and previously as the co-emperor of the Empire of Nicaea from 1259 to 1261. Michael VIII was the founder of the Palaiologan dynasty that would rule the Byzantine Empire until the Fall of Constantinople in 1453. He recovered Constantinople from the Latin Empire in 1261 and transformed the Empire of Nicaea into a restored Byzantine Empire. His reign saw considerable recovery of Byzantine power, including the enlargement of the Byzantine army and navy. It also included the reconstruction of the city of Constantinople, and the increase of its population. His re-establishment of the University of Constantinople contributed to the Palaeologan Renaissance, a cultural flowering between the 13th and 15th centuries.

It was also at this time that the focus of the Byzantine military shifted to the Balkans, against the Bulgarians, leaving the Anatolian frontier neglected. His successors could not compensate for this change of focus, and both the Arsenite schism and two civil wars which occurred from 1321–1328 and 1341–1347 undermined further efforts toward territorial consolidation and recovery, draining the empire's strength, economy, and resources. Regular conflict between Byzantine successor states such as Trebizond, Epirus, Bulgaria and Serbia resulted in permanent fragmentation of former Byzantine territory and opportunity for increasingly successful conquests of expansive territories by post-Seljuk Anatolian beyliks, most notably that of Osman, later called the Ottoman Empire.

John IV Doukas Laskaris (or Ducas Lascaris) (Greek: Ἰωάννης Δούκας Λάσκαρις, romanized: Iōánnēs Doúkās Láskaris; December 25, 1250 – c. 1305) was the fourth emperor of the Nicaean Empire from August 16, 1258 to December 25, 1261, one of the Greek successor states formed after the Sack of Constantinople by the Roman Catholics during the Fourth Crusade. He was the last emperor from the prominent Laskarid dynasty and the last to only rule Nicaea before the Reconquest of Constantinople by his successor in 1261.

The Latin Patriarchate of Constantinople was an office established as a result of the Fourth Crusade and its conquest of Constantinople in 1204. It was a Catholic replacement for the Eastern Orthodox Ecumenical Patriarchate of Constantinople and remained in the city until the reconquest of Constantinople by the Byzantines in 1261, whereupon it became a titular see with only ceremonial powers. The St. Peter's Basilica was the patriarchium, or papal major basilica assigned to the Patriarch of Constantinople, where he officiated when visiting Rome. The office was abolished in 1964.