Siraj-ud-Dawlah became nawab of Bengal on the death of his granduncle, 'Ali Vardi Khan, in 1756. Faced with opposition to his succession from other family members, he was also troubled by the fortification of Calcutta, undertaken without his permission by the British East India Company, which supported his primary opponent, Rajballabh. Although Siraj-ud-Dawlah succeeded in removing threats from rival claimants, the British governor of Calcutta continued to defy his requests to cease fortifying the city.

Convinced that the British would not comply, Siraj-ud-Dawlah marched on the city, taking the English post at Cossimbazar along the way. Shortly after he arrived, on June 16, 1756, the governor, the majority of his staff, and a number of British inhabitants fled Fort William for the safety of English ships in the harbour. After putting up a weak resistance, the fort surrendered on June 20, and on that night there occurred the "Black Hole" incident.

Calcutta was retaken by Robert Clive and Admiral Charles Watson on Jan. 2, 1757. The British reinforced their position of power by plotting with Mir Ja'far, Siraj-ud-Dawlah's general, to overthrow the nawab. After alienating the Hindu bankers and his army, Siraj-ud-Dawlah fell victim to their treachery at Plassey, where on June 23, 1757, Clive, with an army of 3,000, defeated the nawab and his army of 50,000. He fled to Murshidabad but was captured and executed.