The Nestorian Church traces its existence back to Nestorius, fifth-century bishop of Constantinople, who formulated a doctrine that Christ was predominantly human in nature. Although this was declared a heresy by the Council of Ephesus in 431, the faith flourished, and Edessa (Şanlıurfa), Antioch (Antakya) and Nusaybin near Mardin became important Nestorian centres. After Mongol attacks, the Nestorians fled to the Zagros mountains of western Iran and the wild mountains around present-day Hakkari. Isolated in the inaccessible mountains, they developed their own ethnic as well as religious identity, with half the population organized into tribes little different from their Kurdish neighbours.
Serious rivalries developed during the nineteenth century between the Nestorians and the Kurds, exacerbated by the British and American missionaries who were proselytizing in the region. Local Kurdish leaders massacred many Nestorian men around Hakkari, and sold the women and children into slavery. In 1915 the Nestorian patriarch sided with the World War I Allies. After the war the Nestorians, now seen as traitors, fled to Iran and then Iraq, though half their number perished in the exodus. A short-lived attempt to resettle their mountain fastnesses was crushed by Atatürk in the early years of the Turkish Republic. Today just a few tens of thousands survive in Iraq, Iran and Syria, with the patriarchate now in Chicago.