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The causes for the Revolt of 1857 or First War of Independence were: 

1. Political causes: The Doctrine of Lapse which was introduced by the British stated that any kingdom of India which did not have its natural heir the kingdom would pass on to the hands of the British. Many states were annexed under this Doctrine. This was why Laxmibai's son who was adopted was denied the throne. 
2. Economic cause: In order to establish their monopoly over India, British employed landlords or jamindars of the Ryotwari Settlement and Mahalwari settlement. They were merciless and they forced the Indian farmers to do what they said. 
3. Social causes: Christianity arose in India as the Britishers had been established there. So there were many Hindus who transformed themselves into Christians. This hurt the Hindus as they left their own natural caste. Also the abolition of certain reforms such as sati, child marriage,etc. was introduced. This hurt the Hindu sentiments and they thought that the Britishers were removing the Hindu caste and disregarding their religion. 

4. Military cause: The discrimination in the British army between Indians and Britishers were great. All the Indian soldiers were treated with great contempt. While the British soldiers got up to high posts an Indian soldier could not rise above the post of a Subedar. 
5. Immediate Cause: The Enfield Rifle: A new rifle was introduced in The British army. The cartridge of the rifle had a greased paper which was to be bitten off then to reload the rifle. It was known that the greased paper was made from the fat of the cows and pigs. This angered the Indians who just needed a spark to ignite their anger and this was provided by this incident. Mangal Pandey an Indian Sepoy was hanged because he refused to open the greased paper of the cartridge.