Also bangladesh was always part of India by that fact even i believe bangladesh should be a part of India...
We fought a long and prolonged war against the Delhi Mughals. It started in 1526 when the Mughals took over Delhi and the ruling Muslim Pathans fled to Bengal. Bengal army under Sher Shah Suri took over Delhi in 1539, but was ousted in 1556 by Humayun in the Battle of 2nd Panipath.
There was relative peace until 1576, when Pathan Sultan of Bengal was defeated in the Battle of Rajmahal. However, the war continued until 1605, when Bengal went under Delhi. So, Bengal was not always a part of Delhi. It was forcefully annexed to Delhi after almost an 80 year long war.
But, Orissa, Jharkhand, Bihar and west Bengal was part of Sultanate of Bengal all along. Assam could not be conquested, but muslims from Bengal always migrated to this land since after a failed invasion in 1205 by Ikhtiar-ud-Din Muhammad Bakhtier Khilji. Assam was more a part of greater Bengal than it ever was a part of India or Hindustan.