Which battle of 1071 opened Anatolia to the Seljuk Turks?

On 26 August 1071, near Manzikert in what is now eastern Turkey, the Seljuk sultan Alp Arslan defeated a Byzantine army and captured its emperor, Romanos IV Diogenes. Alp Arslan, by most accounts, treated his prisoner with courtesy, agreed terms, and let him go. The battle itself was not especially bloody by the standards of the age.

The catastrophe was what happened next, and it was entirely Byzantine. Romanos returned to find he had been deposed; he was defeated, blinded, and died of the wounds. The rival factions then spent a decade fighting each other, and every soldier used in that fighting was a soldier not defending the Anatolian plateau. Turkmen groups moved into the vacuum and settled it. Within ten years the Seljuks of Rum held Nicaea, about ninety miles from Constantinople.

Anatolia had been the empire's recruiting ground and its granary, and losing it turned Byzantium from a power into a survivor. The appeals for Western help that followed led, by a long and crooked route, to Pope Urban II's call at Clermont in 1095. It is the standard example of a defeat that mattered less for what it destroyed than for what it exposed.