Russia must stop the Turkification of Iranians, and return Iran to Iranians. Russia must stop the Turkification of the Caucasus, and return it to Caucasians.
Why did the Azeri people shift from speaking a Northwestern Iranian language to the Turkic language we all know today?
Azeri was the dominant language in Azerbaijan before it was replaced by Azerbaijani, which is a Turkic language.
This language was known as Old Azeri or Azari and had affinities to the Talysh languages that are spoken nearby. Many linguistic islets are said to persist into the 19th Century while certain Iranian dialects that is still spoken in the area is said to be descendants of the former language.
Āḏarī (Ar. al-āḏarīya) was the Iranian language of Azerbaijan before the spread of the Turkish language, commonly called Azeri, in the region.
Turkic Languages do not originally belong to west of Caspian Sea but central Asia. People of west Turkey and northwest of Iran and Azerbaijan countries (roughly Anatolian plateau) were once speakers of Iranian, European, Semitic, Caucasian, Slavic and some other Languages. Some of them first abandoned their native languages in favor of Greek due to Hellenization.
The process you seek is called Turkification and it began with Turkic Migration from central Asia to Middle-east and Europe. Circa 11th century, war with Turks began and led to deaths of many natives, while others were enslaved and removed. As areas became depopulated, Turkic nomads moved in.
Later in Ottoman era there happened Armenian Genocide, Greek Genocide and Assyrian Genocide due to Turkification. Many were compelled but later some chose Turkish willingly because it became the predominant language and lingua franca of the area.
Most academics view the linguistic Turkification of predominantly non-Turkic-speaking indigenous peoples and assimilation of small bands of Turkic tribes as the most likely origin for the Azerbaijani people.
The Turkic Azerbaijani language only began replacing the Iranian Old Azeri language with the advent of rule of the Safavid dynasty, when hundreds of thousands of Qizilbash Turkic peoples from Anatolia arrived into Azerbaijan, being forced out by the Ottoman Sultan Selim I with more to follow. Earlier, many Turkic speaking nomads had chosen the green pastures of Azerbaijan, Aran and Shrivan for their settlement as early as the advent of the Seljuq dynasty. However, they only filled in the pasturelands while the farmlands, villages and the cities remained Iranian in language and culture. The linguistic conversion of Azerbaijan went hand in hand with the conversion of the Azeris to Twelver Shia Islam. By the late 1800s, the Turkification of Azerbaijan was near completion, with Iranian speakers found solely in tiny isolated recesses of the mountains or other remote areas (such as Harzand, Galin Ghuya, Shahrud villages in Khalkhal and Anarjan).