The immediate trigger was the introduction of the Enfield rifle in 1857. Its cartridges had to be bitten open before loading, and a rumour spread that they were greased with the fat of cows and pigs — anathema to both Hindu and Muslim sepoys. The British authorities dismissed the fear contemptuously and pressed the sepoys to use the cartridges.

Matters came to a head at Barrackpore, where Mangal Pandey refused the cartridge and attacked his officers on 29 March 1857. He was executed, and his regiment was disbanded. On 9 May 1857 at Meerut, 85 sepoys were court-martialled and sentenced to ten years' imprisonment for refusing the greased cartridges; the very next day the entire Meerut garrison rose, released their comrades, and marched on Delhi.