What started the Indian Mutiny?

The immediate cause for the Indian Mutiny of 1857 was the introduction of cartridges lubricated with the fat of cows and pigs, a practice offensive to Hindus and Muslims. Coupled with many social and economic discriminationon part of the British, this is the final straw for the Indians

For further reading check the following articles:

• Revolt of 1857, political and Economic Factors
• Revolt of 1857: Immediate Factor
• Revolt of 1857 Social Causes