The Turks, who were nomadic pastoralists, inhabited central Asia, but the environment began to change, and there was very little rainfall that came. This led to no flooding of the river, which sabotaged their lifestyle's norms.
As these environmental changes got worse and worse, they could no longer sustain a good lifestyle, so they had to find somewhere else to inhabit.
These Turkish nomadic pastoralists, also known as the Seljuk Turks(Sunni Muslims, left in search of better lands, travelling through and over steppe lands.
As they moved south and west into India, they wreaked havoc politically and economically. The Shiites, at the time, were in control of Baghdad(as well as Egypt and much of North Africa). Eventually they forced the Shiite family that was controlling Baghdad out and took over the city.
The Sunni Turks took over the Iranian plateaus and ended the flourishing of culture that had been happening in the early parts of the 11th century. They also officially took Baghdad in 1055 and made Mesopotamia a nomadic state, because the Abbasids couldn't defend against the Turk military
This ruling broke Islamic land into 3 parts. In the east,central Asia, Iran, and east Iraq, the Muslim Caliphates from Abbasid rule wanted to represent all of Islam, but they deferred the rule of the Turkish military.