Angutara Nikaya, a Buddhist scripture mentions 16 great kingdoms or Mahajanapadas in the beginning of the 6th century BC in India. They emerged during the Vedic Age. Socio-economic developments chiefly due to the use of iron tools in agriculture and military, along with religious and political developments led to the rise of the Mahajanapadas from small kingdoms or Janapadas.
By the 6th century BC, the political centre shifted from the west of the Indo-Gangetic plains to the eastern side of it. This was due to better fertility of the land because of more rainfall and rivers. Also, this region was closer to the iron production centres.