Erivan Khanate
The area to the north of the river Aras, amongst which territory lies the contemporary Republic of Azerbaijan, was Iranian territory until Russia occupied it in the 19th century. About a decade later, in violation of the Gulistan treaty, the Russians invaded Iran's Erivan Khanate. This sparked the final bout of hostilities between the two, the Russo-Persian War of 1826–1828. The resulting Treaty of Turkmenchay, forced Qajar Iran to cede sovereignty over the Erivan Khanate, the Nakhchivan Khanate and the remainder of the Lankaran Khanate, comprising the last parts of the soil of the contemporary Azerbaijani Republic that were still in Iranian hands. After the incorporation of all Caucasian territories from Iran into Russia, the new border between the two was set at the Aras River, which, upon the Soviet Union's disintegration, subsequently became part of the border between Iran and the Azerbaijan Republic.