Introduction:
The Indian Rebellion of 1857, also known as the Sepoy Mutiny, was a significant event in Indian history that marked the beginning of the end of British colonial rule. It was a widespread uprising against British rule by Indian soldiers, known as sepoys, in the British East India Company's army. While several acts during the rebellion influenced its transformation from a mutiny to a revolutionary war, the single act that played a crucial role was the proclamation of Bahadur Shah II as the Emperor of India.

Bahadur Shah II's Proclamation:
- The proclamation of Bahadur Shah II as the Emperor of India had significant symbolic and political implications.
- Bahadur Shah II was the last Mughal emperor and held immense historical and cultural significance in India.
- By proclaiming him as the Emperor of India, the rebels sought to unite the diverse Indian population under a single leadership and rally them against British rule.
- This act gave the rebellion a broader objective of overthrowing British rule and restoring Indian sovereignty, transforming it from a localized mutiny into a pan-Indian struggle for independence.

Impact on the Rebellion:
- The proclamation of Bahadur Shah II gave the rebellion legitimacy and helped mobilize support from various sections of society.
- It appealed to the sentiments of Indian soldiers, who saw themselves as defenders of their religion and culture under the leadership of the Mughal emperor.
- The proclamation also resonated with the Indian civilian population, as it represented a symbol of national unity and a call for independence.
- The rebellion spread rapidly throughout northern and central India, with rebels capturing major cities and regions.
- The rebels formed alliances with local rulers, peasants, and other disaffected groups, creating a broad-based resistance against British rule.
- The rebellion turned into a full-fledged revolutionary war, with intense battles fought between the rebels and the British forces.
- The British responded with brutal force, leading to widespread atrocities and acts of violence from both sides.
- The rebellion ultimately failed to achieve its objectives, but it had a profound impact on Indian nationalism and paved the way for future independence movements.

Conclusion:
The proclamation of Bahadur Shah II as the Emperor of India was the single act that transformed the sepoys' mutiny into a revolutionary war. It gave the rebellion a broader objective of overthrowing British rule and uniting diverse sections of Indian society under a common cause. While the rebellion ultimately failed, it laid the foundation for future struggles for independence and played a crucial role in shaping Indian nationalism.