King Asoka became Buddhist after he was horrified with the tyranny of Kalinga War. It was in:

The Mauryan Empire's Ashoka and the Kalinga Kingdom fought the Kalinga War. Ashoka is regarded as one of India's greatest monarchs, reigning from 273 and 232 BC. The Kalinga War, which took place in 261 BC, was a watershed moment in his life. The war resulted in massive destruction, with over a million combatants and civilians slain. Following the war, Ashoka rejected violence and adopted buddha's doctrine of ahimsa (non-violence).