Around 1400 , the Turco-Mongol nomadic conqueror Timur arose out of the Eurasian Steppe to lead military campaigns across Western , Southern and Central Asia , ultimately seizing much of the Muslim world after defeating the Mamluks of Egypt and Syria , the emerging Ottoman Empire , and the declining Delhi Sultanate . Timur 's conquests devastated most Assyrian bishoprics and destroyed the 4000-year-old city of Assur , which was the cultural and religious capital of the Church of the East and its followers . After the destruction brought on by Timur , the massive and organized Nestorian Church structure , which at its peak extended as far as China , Central Asia , Mongolia and India , was largely reduced to its region of origin ( with the exception of the Saint Thomas Christians in India ) , and stayed as such until the Assyrian genocide , when a large portion of this region was entirely , ethnically and culturally cleansed of its endemic population , and in effect also ended the Shimun Branch , which had to reestablish itself in America up until 2015 when they established their new see in Erbil . Along with the destruction of the Hakkari cultural region , the Assyrians of Tur Abdin , Amid , Urfa and other regions of the southeast suffered genocide as well , but due to an agreement with the Turks , the Syriac Orthodox Church was able to exist in the region after the end of the genocide , and a Syriac community still exists in Turkey until this day , and is the most geographically spread out Church still functioning in Turkey , with active churches in Adiyaman , Siirt , Istanbul , and its primary area of operation and seat at Mor Gabriel Monastery in Tur Abdin
Assyrian genocide
Assyrian genocide
Chaldean Catholic Church