What was the immediate cause of the First War of Independence?

The immediate cause of the First War of Independence was the introduction of the Enfield rifle. The loading process of the Enfield rifle involved bringing the cartridge to the mouth and biting off the top greased paper with the teeth.

In January 1857, there was a rumour in the Bengal regiments that the greased cartridge had the fat of cow or pig. The sepoys were now convinced that the introduction of greased cartridges was a deliberate move to defile Hindu and Muslim religions. So, the soldiers refused to use these cartridges and staged an uprising when they were forced to use them.