When the Republic of Turkey was formed from the hollowed out husk of the Ottoman Empire in 1923, one of the problems that its new leaders, including or maybe particularly Mustafa Kemal Atatürk (a surname that was given to him later in life, which means “father of the Turks”), was how to quickly develop a Turkish national identity to energize this new nation-state. There had been movements in favor of embracing the “Turkic” nature of the Ottoman Empire, but these never went anywhere because the emperors and their ministers were trying (and failing, for the most part) to keep the empire together and retain the loyalties of a vast array of peoples, most of whom were not Turkish. Meanwhile there were nationalist movements developing all over the empire in the 19th century, especially in its European provinces (Greece and the Balkans), and later, not fully until World War I, among its large Arab population. But for the most part there was no serious Turkish nationalist movement, because the Turks who ran the empire realized that nationalism was the last thing they wanted to be pushing.

Mustafa Kemal, “Father of the Turks”
After World War I and the collapse of the empire, though, with the empire’s geographic core (and most “Turkish” parts) being reorganized as the new Republic, Turkish nationalism became kind of a Big Deal. Atatürk recognized that a strong national identity was crucial to controlling his new state and keeping it together, not to mention how important it would be in terms of protecting Turkish interests in the post-war geopolitical order in Europe. His new government at Ankara, which had bitterly fought for and won recognition of Turkey’s nationhood during the 1919-1923 Turkish War of Independence, needed to demonstrate that the new Republic could hold together or risk European encroachment. One area where Atatürk and his supporters felt that big gains could be made in defining a uniquely Turkish identity was in the Turkish language. Because Arabic script was still hell for printing in the 1920s and because its vowel-light writing system had made it a fairly bad script for representing the vowel-heavy Turkish language, Atatürk instituted script reform in 1928, which is why modern Turkish is written in Latin, not Arabic, script. He also established the Turkish Language Foundation to research the language, primarily in order to purge the many Arabic and Persian words that had been assimilated into the Turkish lexicon and replace them with “original” Turkish vocabulary, either new words derived from Turkish roots or archaic Turkish words brought back into use and given new relevance.
In the long run this worked, and continues to work, quite well–so well, in fact, that Atatürk’s famous speeches, the ones that Turkish schoolchildren learn, have been translated multiple times over the years to make the language more contemporary and more “Turkish.” But in the short run, it had the effect of making the new, modern Turkish language unintelligible for older Turks who were accustomed to using all those Arabic and Persian words in everyday speech. In order to ease the transition while also doing a little pro-Turk ethnic puffery, the School of Language, History, and Geography at Ankara University hit upon the Güneş Dil Teorisi, or “Sun Language Theory,” by which they asserted that every other human language could be traced back to one Central Asian root, which was (obviously, duh), a proto-Turkic language whose closest contemporary relative was, of course, Turkish. There’s pretty much nothing to this theory apart from some French research (not well-regarded anymore) suggesting that Sumerian hieroglyphs and cuneiform were the first writing systems and an assertion that the Sumerians were Turks from Central Asia. These early Sumerian-Turks, the theory goes, started worshiping the sun, as one might do in their situation, and what became the Sumerian-Turkish language started as ritual chanting in praise of their sun god.
The lack of any real scholarly evidence backing the Sun Language Theory up didn’t stop it from being accepted in Turkey–I mean, it was nice to think that Turkish was the root of every other language, and the problem of so much Arabic and Persian vocabulary in the Turkish lexicon was solved, because, hey, all Arabic and Persian words were ultimately Turkish anyway! Turkish linguists created fanciful Turkish etymologies for all sorts of words, like “God” being derived from the Turkish kut (“blessing”) or the idea that the Turkish okul was actually the root of the Greek word σχολείο and the Latin schola instead of, you know, the other way around. There’s apparently a word for the creation of fake etymologies to support loony theories about linguistic origins, which I didn’t know until I started writing this post. It’s called goropism, which comes from a 16th century dude named Johannes Goropius Becanus, who asserted that some dialect of Dutch was the language spoken in Eden because, and as far as I can tell this is his actual justification, it had more short words in it than either Latin or Greek (hey, it must have made sense to him).
The people who were got the worst end of this whole language reform business were, unsurprisingly, Turkey’s Kurds, who simultaneously had their ethnic identity stripped from them (the Turkish government summarily announced that Kurds were just “Mountain Turks,” whatever the hell that means) and had their Kurdish language (which is a heck of a lot closer to Persian than to Turkish, since the Kurds are, in fact, an Iranian people, not a Turkic one) suppressed and banned from public use.