By the year 1857 the British had established complete political control of India. As Western education was introduced and missionaries eroded Hindu society resentment among Indian people grew and it was joined by unease among the old governing class when the British decided to formally abolish the Mughal Empire.
The mutiny of the Sepoy (= native troops in the British army) began on May 10, 1857, when Indian soldiers who had been placed in irons for refusing to accept new cartridges were rescued by their comrades. The greased cartridges had to be bitten off before use, and the manufacturers had supplied a fat of beef and pork - repulsive to both Hindus and Moslems.
The Indian garrison at Delhi joined the mutineers and proclaimed Bahadur Shah, the titular Mughal emperor as their leader.The capture of Delhi turned the mutiny into a wide-spread revolt. But the leaders were not united, because they sought to revive former Hindu and Muslim regimes, which traditionally had been opposed to each other.
The British had some advantages. They did receive reinforcements from Britain, and they had a base in Bengal, and in the Punjab the Sikhs were so hostile to the Muslims that they supported the British against the Mughal restoration in Delhi.
The British recaptured Delhi in the fall of 1857, further campaigns followed the next year and by the spring of 1858 the British were again in full control of India.