Background
Following the Fourth Crusade, the Byzantine Empire had fractured into the Greek successor-states of Nicaea, Epirus and Trebizond, with a multitude of Frankish and Latin possessions occupying the remainder, nominally subject to the Latin Emperors at Constantinople. In addition, the disintegration of the Byzantine Empire allowed the Bulgarians, the Serbs and the various Turcoman emirates of Anatolia to make gains. Although Epirus was initially the strongest of the three Greek states, the Nicaeans were the ones who succeeded in taking back the city of Constantinople from the Latin Empire
The Nicaean Empire was successful in holding its own against its Latin and Seljuk opponents. At the Battle of Meander Valley, a Turkic force was repelled and an earlier assault on Nicaea led to the death of the Seljuk Sultan. In the west, the Latins were unable to expand into Anatolia; consolidating Thrace against Bulgaria was a challenge that kept the Latins occupied for the duration of the Latin Empire.
In 1261, the Empire of Nicaea was ruled by John IV Laskaris, a boy of ten years. However, John IV was overshadowed by his co-emperor, Michael VIII Palaiologos. Palaiologos was a leading noble of military standing and the main figure of the regency of John IV, who had used this role to propel himself to the throne, and set the stage for his becoming sole Emperor of the restored Byzantine Empire.
Restoration of the Byzantine Empire
In 1259 CE, Michael VIII came to the throne of the Empire of Nicaea. He founded the Palaiologos Dynasty, the longest and last dynasty of Byzantine rulers. In 1261 CE, Michael's forces succeeded in capturing Constantinople while the Latin knights were off fighting elsewhere. They found the city a shell of its former self, sparsely populated and largely ruined. Still, Michael VIII returned to the city and was proclaimed emperor there, marking the restoration of the Byzantine Empire.
Byzantine Coin
A gold Byzantine coin, called the hyperpyron (which replaced the earlier solidus), depicting the first emperor of the Byzantine Palaiologan Dynasty, Michael VIII.
In order to protect his empire from further attacks by Western knights, he attempted to end the schism between the Catholic and Orthodox churches. This outraged many of his citizens, who blamed the Catholics for the sack of Constantinople. A decisive change had taken place: among the citizens of the restored Byzantine Empire, the "Latins" of Western Europe were more hated than even the Muslims.
The war-ravaged empire was ill-equipped to deal with the enemies that now surrounded it. In order to maintain his campaigns against the Latins, Michael pulled troops from Asia Minor, and levied crippling taxes on the peasantry, causing much resentment. Massive construction projects were completed in Constantinople to repair the damages of the Fourth Crusade, but none of these initiatives was of any comfort to the farmers in Asia Minor, suffering raids from fanatical ghazis.
As a result, Anatolia, which had formed the very heart of the shrinking empire, was systematically lost to numerous Turkic ghazis, whose raids evolved into conquering expeditions inspired by Islamic zeal. With a decreasing source of food and manpower, the Palaiologoi were forced to fight on several fronts, most of them being Christian states: the Second Bulgarian Empire, the Serbian Empire, the remnants of the Latin Empire and even the Knights Hospitaller.
The loss of land in the east to the Turks and in the west to the Bulgarians was complemented by two disastrous civil wars, the Black Death and the 1354 earthquake at Gallipoli, whose destruction and evacuation allowed the Turks to occupy it. By 1380, the Byzantine Empire consisted of the capital Constantinople and a few other isolated exclaves, which only nominally recognized the Emperor as their lord. Nonetheless, Byzantine diplomacy coupled with the adroit exploitation of internal divisions and external threats among their enemies, and above all the invasion of Anatolia by Timur, allowed Byzantium to survive until 1453.
The Palaiologan Renaissance
During the Palaiologan Dynasty, the empire experienced a short but vibrant renaissance, known as the Palaiologan Renaissance. As the Palaiologan emperors attempted to restore the glory of Constantinople, they sponsored art and encouraged philosophy. Artists and philosophers looked to the classical past and rediscovered much ancient learning. Although the Palaiologan Renaissance came too late to save the struggling Byzantine civilization, it would be a major catalyst for the Italian Renaissance, especially as Byzantine artists and scholars traveled to Italy to seek shelter from the new threats that besieged the empire.
Towards the 14th century, as the Empire entered into a phase of terminal crisis, such achievements became less valued. All was not lost for these seemingly rejected scholars — many in Italy who had been opened up to Byzantium by the maritime expansions of Genoa and Venice came to appreciate their achievements, facilitating the Renaissance. As such these scholars found themselves in Italian institutions, expressing their Greco-Roman culture for pay. Immigration to Italy was made less attractive by the idea of abandoning the Orthodox faith to practice Catholicism. Nonetheless, a significant and increasing number of Greeks began travelling to Italy, first temporarily to Italian colonies such as Crete or Cyprus before returning to Byzantium, then as the Empire began to fail horribly, in more permanent manner. The Fall of Constantinople was marked by large amounts of Greek refugees escaping Turkic rule into Europe via Italy and thus accelerating the Renaissance.