Why did the Seljuq Turks threaten the Byzantine Empire?

The Seljuq Turks posed a significant threat to the Byzantine Empire primarily through their expansion into Anatolia, which served as the empire's military and economic heartland.12

This threat developed through several key factors:

• Turkmen Incursions: Originally, irregular Turkmen warriors began raiding Byzantine borderlands in Armenia and Anatolia. Great Seljuq sultans, such as Toghrïl Beg and Alp-Arslan, diverted these lawless elements into Byzantine territory to stabilize their own domains and provide land for the warriors.1
• Military Defeat: The Battle of Manzikert in 1071 was a turning point. The defeat of the Byzantine army and the capture of Emperor Romanus IV Diogenes shattered the empire's eastern frontier defenses.13
• Internal Instability: Following Manzikert, Byzantine civil wars and the neglect of frontier defenses allowed the Turks to plunder the countryside and isolate cities. By 1081, they had reached Nicaea, bringing a resurgent Islam perilously close to Constantinople.24

This existential danger prompted Emperor Alexius I Comnenus to seek military aid from the West, ultimately leading to the First Crusade.43