I. Prelude: The Crisis of Bengal (1756 AD)

Upon his accession in 1756 AD, the young Nawab Siraj-ud-Daulah demanded that the English East India Company cease their illegal fortifications in Calcutta. The Company's refusal led to the Nawab’s capture of Fort William in June 1756 AD, an event famously associated with the 'Black Hole Tragedy'—a narrative used by the British to justify their subsequent aggression.

Admiral Watson and Colonel Robert Clive were dispatched from Madras, retaking Calcutta in January 1757 AD. The Treaty of Alinagar (February 1757 AD) briefly restored peace, but Clive knew that as long as Siraj-ud-Daulah remained Nawab, the Company’s trade would be at risk. The conspiracy to replace him was born.

II. The Battle of Plassey: The Great Betrayal

Clive marched toward the capital, Murshidabad, with only 3,000 men. He faced the Nawab’s army of over 50,000. However, the battle was decided before a single shot was fired. Clive had entered into a secret pact with Mir Jafar (the Nawab's Commander-in-Chief), Jagath Seth (a powerful banker), and Omichund.

During the battle, Mir Jafar’s wing of the army remained stationary. Only a small contingent under Mir Madan and Mohan Lal fought bravely. A sudden rainstorm dampened the Nawab’s gunpowder; the British, who had kept their powder dry with tarpaulins, decimated the remaining loyalists. Siraj-ud-Daulah fled and was later executed, making June 23, 1757 AD the date Bengal fell into the Company's pocket.

III. Transition of Power: From Mir Jafar to Mir Qasim

Mir Jafar was installed in 1757 AD, but the Company's insatiable demand for money led to his removal. In 1760 AD, his son-in-law, Mir Qasim, was made Nawab. Qasim was efficient and ambitious; he shifted his capital to Munger and began modernizing his army with European-style training.

Conflict arose over the misuse of Dastaks (trade permits). Mir Qasim abolished all duties for Indian merchants to level the playing field, which infuriated the British. After several minor defeats in 1763 AD, Mir Qasim fled to Awadh to form a Grand Alliance.

IV. The Battle of Buxar: The Decisive Victory

Unlike Plassey, Buxar was a true test of military discipline. Mir Qasim joined forces with Shuja-ud-Daulah (Nawab of Awadh) and Shah Alam II (The fugitive Mughal Emperor). They faced a disciplined Company force under Major Hector Munro.

Munro’s forces utilized superior artillery and infantry drills. The disorganized allied forces were decisively beaten. This victory proved that the Company's military system could defeat the combined might of the major North Indian powers. Buxar made the Company the de facto rulers of Northern India.

V. The Legacy: Treaty of Allahabad

Robert Clive, returning as Governor, signed the Treaty of Allahabad in 1765 AD. The Mughal Emperor granted the Company the Diwani of Bengal, Bihar, and Odisha. This was the moment the Company stopped being a merchant and started being a state. They now controlled the revenue of millions, which they used to fund further conquests across the subcontinent.