It is important to note that the Eastern Roman Empire in 1453 was absolutely a shadow of its former self. Imperial territories in Anatolia had been gradually reduced following the Turkic conquests of the 11th century and threatened the Empire’s position in Europe. After the Seljuk Turks were defeated by the Mongols, they became vassals of the Ilkhanate, and the Seljuks disintegrated with the Ilkhanate, leaving a number of independent Turkish Beyliks vying for control in Anatolia, of which the Ottomans would become preeminent. The loss of Anatolia was incredibly detrimental to the finances of the Eastern Roman Empire, as it had incredible material wealth and supplied large amounts of manpower for the Imperial Army. Additionally, civil strife within the Empire significantly destabilized the state, as the bureaucracy and army vied for power.

Critically, the 4th Crusade devastated the Empire. A Crusader army led by the Venetians sacked Constantinople. This devastated the city and a large number of its inhabitants were killed. After seizing Constantinople, the Crusaders established a short-lived Latin Empire which was surrounded by enemies. The Empire was also partitioned as a result of the Crusade and several Greek rump states were formed. One of these states, the Nicene Empire, would restore the Eastern Roman Empire. The newly-restored Empire would find little peace, as it was attacked on various fronts by many enemies. The Black Death compounded these misfortunes, as it killed over half the population of Constantinople.

By the time of the Black Death the Ottoman Empire had been working to establish itself as a regional force. Murad I conquered significant amounts of territory in the Balkans and further isolated the weakened Eastern Roman Empire. For a time, the Eastern Roman Empire was a vassal of the rising Ottoman state. Ottoman expansion in the Balkans was temporarily halted by their defeat at Ankara in 1402 by the army of Timur, giving a brief respite to the Eastern Romans. The Ottomans would rapidly recover, however, and once again established themselves as a regional power.

By 1453, Eastern Roman territories had been reduced an incredible amount, and Constantinople was isolated and depopulated. Mehmed II’s construction of a fort on the European side of the Bosporus isolated the city even further. The Ottomans therefore controlled both the Dardanelles and the Bosporus, which were key chokepoints for trade and military traffic.

Eastern Roman economic decline, as well as their general decline, can therefore be attributed to a number of factors. The loss of Anatolia had disastrous implications for their control over trade and also deprived the Empire of a key source of resources and manpower. This was compounded by the 4th Crusade, which fractured the Empire and depopulated Constantinople. The Black Death furthered Imperial misfortunes, and an expanding Ottoman polity set its sights on the remains of the Empire. The Ottomans isolated the Empire and ultimately ended it.

SOURCES:

Peter Charanis, Economic Factors in the Decline of the Byzantine Empire

Caroline Finkel, Osman’s Dream, the History of the Ottoman Empire

Chris Wright, On the Margins of Christendom: The Impact of the Crusades on Byzantium

For further reading about the state of Byzantium and Anatolia from the 11th-15th centuries, I would recommend:

ACS Peacock, The Great Seljuk Empire

Michael Angold, The Fourth Crusade, Event and Context

Joseph Gill, Church Union: Rome and Byzantium 1204-1453