Th east India company gained the control over Bengal by fighting a war.
This war was fought between the nawab of Bengal and Robert Clive( the governor general) in which the commander of the Bengal's army signed a deal with Robert Clive saying that he won't train the army
in a reward after the defeat of Bengal by Robert Clive and his army he was made as the next nawab.
Hope u can understand!!
Treaty of Allahabad ( 16 August 1765), it was the outcome of the Battle of Buxar (1764). Robert Clive (Governor General of Bengal) made a separate treaty with Mughal Emperor Shah Alam II and Nawab of Awadh Shuja-ud-daullah. A/c to this treaty, Mughal Emperor granted Fiscal Rights (Diwani) to the East India Company at Bengal, Bihar and Orissa.