Why did Byzantine rule in Anatolia decline after the Fourth Crusade?

Following the Fourth Crusade and the sack of Constantinople in 1204, the Byzantine Empire fragmented, leading to the rise of the Empire of Nicaea in northwestern Anatolia. While this state successfully maintained a modus vivendi with the Seljuqs and eventually recaptured Constantinople in 1261, this victory proved disastrous for Byzantine control in Asia.1

The decline of Byzantine rule in Anatolia after 1261 was driven by several critical factors:

• Neglect of Asian Provinces: With the restoration of the capital, imperial attention shifted back to Constantinople, leading to the neglect of Anatolian territories just as Mongol pressure weakened Seljuq hegemony. This allowed nomadic Turkmen tribes unrestricted access to poorly defended Byzantine districts.12
• Military Resource Allocation: Emperors, particularly Michael VIII, prioritized defending against Western threats—such as Charles of Anjou—over securing the eastern frontier. Frontier troops were withdrawn to Europe or left unsupported.2
• Economic and Structural Decay: The Byzantine military system had weakened; the pronoia system, which once provided soldiers, had devolved into hereditary estates where landlords ignored military obligations. Consequently, the state relied on expensive hired troops, which crippled the economy and made it impossible to defend both European and Asian territories simultaneously.2

By 1300, the interior was lost, and by 1337, the last major Byzantine holdings in Bithynia had succumbed to independent Turkmen principalities.1