When the Seven Years' War broke out, owing to their constant rivalry with the French, and the fall of Madras to the forces of Dupleix, early in 1756 the British authorities in Calcutta began repairs to the fortifications of old Fort William, which were extremely decayed. This irritated the new Nawab of Bengal, Siraj Ud Daulah, who viewed it as a threat to his sovereignty. Enraged still further when the British granted asylum to one Krishnaballav, who had embezzled money from the dewani of Dhaka, Siraj ud-Daula first attacked and captured Cossimbazar, and then Calcutta, which fell after a short siege on 20 June 1756, during which the Governor and many other officials escaped down the Hooghly River, leaving the remainder of the garrison and the Eurasian population of Calcutta to their fate. This is now known as the Siege of Calcutta. It is said that 123 Britons later died in the Black Hole of Calcutta after his victory, but recent evidence calls into question the numbers involved, and suggests that the Nawab himself was probably unaware of what transpired. He renamed Calcutta Alinagar after the previous Nawab, and his maternal grandfather, Alivardi Khan. Having installed Manikchand as the ruler of Alinagar, Siraj returned to Murshidabad. Soon (on 2 January 1757) Watson and Robert Clive retook Calcutta with a force of Company sepoys and the assistance of the Royal Navy. Hearing the news, Siraj ud-Daula moved to attack Calcutta, but fearing an attack from Ahmad Shah Abdali, after a few days of war he signed the Treaty of Alinagar with the East India Company, giving them permission to build the fort.
Although Siraj ud-Daula conceded temporary defeat in the Pact of Alinagar, he once again began scheming with the French against the British. Meanwhile, the Third Carnatic War was starting in the south. Also at this time, nobles such as Jagat Seth, Mir Jafar, Rai Durlav, Omichand and Rajballav were plotting against Siraj ud-Daula (a principal reason being the Nawab's arrogance, well attested to in contemporary sources) and they invited Clive to take part in their plans. Clive seized on this plan to get rid of two enemies at once. Citing non-existent reasons, he attacked Murshidabad, having previously reached an agreement with Mir Jafar to install him on the musnud of Bengal. On the fateful day of 23 June 1757, 23 miles away from Murshidabad in the mango groves of Palashi, the armies met at the Battle of Plassey. The British army consisted of 800 European soldiers and 2,200 Indian soldiers, while the Nawab's army was made up of 18,000 cavalry and 50,000 infantry. At the start of this seemingly impossible battle, generals Rai Durlav and Iar Latif held their armies together, but in an act of treachery Mir Jafar led his troops away from the battlefield, and the remaining army led by Mirmadan and Mohanlal was defeated. Siraj ud-Daula escaped but was later caught and killed by Miran, the son of Mir Jafar. Mir Jafar was made the new Nawab, and the British had effectively seized control of Bengal. In 1765, after defeating the next Nawab, Mir Qasim, the Nawab of Oudh and the Mughal Emperor at the Battle of Buxar, there was no one to stand in the way of the British and their dominance in North India. Thus, British imperialism began in India with the conquest of Bengal, a game in which a main pawn was the great city of Calcutta.
Calcutta also had an indirect but important influence on the battles of the Carnatic Wars. When Madras fell to Dupleix, the British were still able to direct the war from another of their strongholds, Calcutta. They also used the wealth of Bengal to defeat the French. As Dr. R. C. Majumdar stated in An Advanced History of India, "The Battle of Plassey may be truly said to have decided the fate of the French in India."