Koch Bihar (1586—1949) was a small kingdom located south of the Himalayan kingdom of Bhutan, west of the upper Brahmaputra valley Ahom kingdom, and north-east of the Ganges plain and Bengal. It was formed when the Kamata Kingdom under the Koch dynasty split following the death of Nara Narayan in 1586. The eastern portion, Koch Hajo, was soon absorbed by Ahom. The western portion, Koch Bihar, although nominally independent became a vassal state in turn of the Mughal Empire, of Bhutan, the British East India Company, and of the British Raj. It acceded to and merged with India in 1949 and became a part of West Bengal.
Read more about Koch Bihar: Early History: Mughal Dominance (1587-1680), Bhutanese Domination (1680-1772), British East India Company Acquires Control, Later Princes and Notables, Further Reading