History of Bengal 

Before we are going to know about it, We have to know the History of Bengal from past independence. At that time Pakistan and Bangladesh are also a part of India. On 15th August 1947, India got independence from the Colonial rule of the British. But, On the 14th of August 1947, Pakisthan got Independence and separated from India. On the religious view, all the Muslims were agitated for the formation of a separate country for the Muslims. In the leadership of Muhammad Ali Jinnah, all the Muslims were protested against the formation of separate countries. 
After the discussion of the Indian National Congress leaders of Gandhi, Nehru, Sardar Vallabh Bhai Patel. India was divided into two parts as India and Pakistan. Where the majority of the Muslims went to Pakisthan & Hindu and related religions like Buddhists, Sikhs, Parsi, and Jain were stayed in India.

After the division of the country into two parts present Pakisthan and the East part of the Bengal were formed as Pakisthan. So, the east part of the Bengal is out of India, and the West part of the Bengal is in India. That west part of the Bengal is called as West Bengal. 

After some years Bangladesh also separated from the Pakisthan.  on 26th March 1971, under the leadership of Sheikh Mujibur Rahman Bagalis got freedom from Pakistan and formed a separate country like Bangladesh.

Hence the large state of Bengal divided and Geographically East Bengal is called Bangladesh and West Bengal is called an Indian state.