VICTORY IS MINE!
So, for a thousand years, with some pretty stunning reversals, the Turko-Hunno-Mongol steppe-people (Turkey, the Soviet Stans, Mongolia) and the Persians fought it out.
Yes, for about four hundred years, it seems as though the Mongols had the utmost power, after all, didn't the Chingis-Khanites reach past Iraq, wasn't Tamerlane ruling Persia, his capital their capital?
Certainly the Timurids (after Tamerlane) ruled Persia, but only after making Persian their language. The same was true with the Mongols in China, who ruled, but could not impose the Mongolian language, or even rule with it, but spoke Chinese (a little more research required on that one, I'm sure of the rest).
Say, about 1600, we want to say the major waves of war in this area came to an end. Where was the final, sustainable boundary? Here is Grousset describing it: "At the moment the empires were crystallizing, Persia was destined to remain to the Persians, as was ethnically fitting, and Turkestan to the Turks." (Grousset, 2005 9th printing, Rutgers U. Press, p. 468). By "Turkestan" here Grousset is refering to the countries which are now Turkmenistan, Uzbekistan, Kirghiz and Kazakhstans.
The large scale brutality of the millenia of Turco-Hunno-Mongol wars seems to pretty much outdo any other in the history of the world. Memorably, Tamerlane was quite fond of huge pyramids of skulls, sometimes tens of thousands of heads from a city he had just taken. But when the pushing and shoving came to an end, the line in the sand was the one demarcated by language.