Bengal in the 18th Century was considered the richest of all Indian provinces. This prosperous & fertile land attracted traders from European countries (East India Company).
Trading companies from Holland, France & England came an established trading centres in Bengal.
Farrukhsiyar, Mughal emperor ordered a farman to the British to trade duty free without paying any taxes. This angered Siraj-ud-daulah and on coming to power he ordered the British to pay taxes. This angered the British & they waged the Battle of Plassey against him. Then he captured the British , the British fought over & took over him & signed the Treaty of Alinagar in which it was stated that the British would trade freely without paying any taxes.
And in this way they established monopoly over in Bengal & then expanded to other states of India.