In my prior post "European Diaspora", I raised the concept that populations displace other populations. In real terms, that Western Europeans were being forced to move West as access to the East was "blocked" by aggressive populations, such as the Turks.
Since the original post, it occurred to me; Where did the Turks come from? Wikipedia to the rescue! Wikipedia has several articles concerning this topic. One "Genetic Origins of the Turkish People" appears inconclusive citing both a strong native Anatolian presence and "that Central Asians have made a 30% genetic contribution."
Another Wikipedia article, "Turkish Diaspora", writes "The main migration of Turkish people to Anatolia occurred at the same time of Turkic migration between the 6th and 11th centuries (the Early Middle Ages), when they spread across most of Central Asia and into Europe and the Middle East. The Seljuk Turks (Selçuk Türkleri) were the first Turkish power to arrive in the 11th century as conquerors, who proceeded to gradually conquer the land of existing Byzantine Empire. In the following centuries the local population began to be assimilated from the emerging Turkic migrants. Over time, as word spread regarding the victory of the Turks in Anatolia, more Turkic migrants began to intermingle with the local inhabitants, which helped to bolster the Turkish-speaking population."
Of particular importance to the narrative above is the Battle of Manzikert. In this particular battle, August 26, 1071, the Seljuq Turk forces led by Alp Arslan inflicted a decisive defeat to the Byzantine forces. According to the article this allowed the mass movement of Turks into Anatolia. By 1453 Constantinople fell to the Turks's, eventually culminating in the Battle of Vienna in 1683.
Battle of Hattin (July 4, 1187) (though 120 years later) was similar to the Battle of Manzikert in its outcome, that these Muslim victories opened the door for continued Muslim expansion. In the Battle of Hattin Muslim armies under Saladin captured or killed the vast majority of the Crusader forces severely crippling the ability of Christians to hold onto the Holy Land. Jerusalem fell to Saladin on October 2, 1187. The Muslims eventually conquered the rest of the middle east.