The Sepoy Rebellion was the uprising of natives (sepoys) of India against their British colonizers. Britain's attempt to "Westernize" India through Christianity, education, social policies, etc. was received as a direct threat to the traditional caste system already in place. The attempted implementations by the British were offensive to the native people whom reacted with surprising violence.
The BBC provides a good starting point for further research and deeper understanding.
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India and Great Britain were the two foes.
India's first was of independence was also called the Sepoy Mutiny because it was a mutiny of the Sepoys in the East India Company Army.