Around 180 BC, Artsakh became one of the 15 provinces of the Armenian Kingdom and remained so until the 4th century. Then it became part of the Sasanian Empire. In the 7th century, the region was conquered by the invading Muslim Arabs. The House of Khachen, then ruled Artsakh until the early 19th century. Persia formally ceded the whole of Karabakh to the Russian Empire by the Treaty of Gulistan in 1813. In April 1920 Azerbaijan was taken over by the Bolsheviks. It was in the Azerbaijan Soviet Socialist Republic. In the 1980s there was a movement to have the autonomous oblast transferred to the Armenian SSR. When Mikhail Gorbachev came to power in Moscow and started campaigns of publicity and democratic reforms at the end of the 20th century, Armenians living in Nagorno-Karabakh sent letters to Gorbachev demanding him to move the autonomous oblast to the control of Armenia. When it was declined, the Armenians started an independence movement.