As a result of the 19th century Russo-Persian wars, the Russians took over the Caucasus, and Iran irrevocably lost control over its integral territories in the region (comprising modern-day Dagestan, Georgia, Armenia, and Azerbaijan), which got confirmed per the treaties of Gulistan and Turkmenchay. The area to the north of the river Aras, among which the contemporary Republic of Azerbaijan, eastern Georgia, Dagestan, and Armenia, were Iranian territory until they were occupied by Russia in the course of the 19th century.
What century did Iran lose the Caucasus to Russia?
the 19th century