Why were Azerbaijanis in the Caucasus separated from Azerbaijanis in Iran?

The separation of Azerbaijanis in the Caucasus from those in Iran was the result of territorial shifts following military conflicts between the Russian Empire and Iran in the early 19th century.21

Specifically, the Treaty of Golestan (1813) and the Treaty of Turkmenchay (1828) established a new border between the two empires. Through these agreements, Russia acquired territories in the Caucasus—including Baku, Shirvan, Ganja, Nakhichevan, and Yerevan—which permanently separated the Azerbaijani-speaking population of the Caucasus from their linguistic and religious compatriots who remained in Iran. The Aras River serves as the geographic boundary that continues to divide these two regions today.231