Assam was ruled by various dynasties during the medieval period - the Pala, Koch, Kachari, and Chutiya. The Ahom kingdom established in the 13th century at the upper Assam plain gradually took control over the entire region. Although there was continuous warfare, the kingdom ruled for over 600 years until the 19th century.
The present day Bangladesh and Assam were part of the British India till 1947. Until the idea of border came in, people have moved across this region freely. The British promoted the migration from the Bengal region to Assam for labour in the tea gardens and later for cultivation in the Brahmaputra valley.
Bengal Presidency (1826):Assam was included as a part of the Bengal Presidency
Chief Commissioner's Province (1874): Assam was separated from Bengal Presidency to for a new province. The Sylhet region of East Bengal is added to the Assam region.
Chief Commissioner's Province (1905): Bengal was partitioned and the East Bengal region was added to the existing Chief Commissioner's Province.
Chief Commissioner's Province (1911): Facing the backlash, the British revoke the the Bengal partition. Sylhet remains with Assam
Assam (1947): Assam post Independence. Sylhet joins Bangladesh. Present day Arunachal Pradesh is added to Assam in 1937