Which Indian emperor spread Buddhism after the Kalinga war?

Ashoka, who conquered Kalinga around 260 BC with enormous slaughter and then, by his own account carved into rock across his empire, was so appalled by what he had done that he renounced conquest. It is the only case in ancient history of a ruler publicly recording remorse for his own victory.

The edicts are the evidence and they are extraordinary documents. Carved on pillars and rock faces across the subcontinent in local languages and scripts, they announce a policy of dhamma - religious tolerance, non-violence, care for animals, medical provision for people and animals, roads and wells, and an instruction that officials should treat subjects as a father treats his children. He also sent Buddhist missions abroad, including to Sri Lanka, which is substantially why Buddhism spread beyond India at all.

He was then forgotten for two thousand years. The Brahmi script the edicts were carved in became unreadable, and Ashoka survived only as a name in Buddhist legend until James Prinsep deciphered the script in 1837 and the pillars started to speak. Modern India took his lion capital as its state emblem and the wheel from his pillars for the centre of its flag - a national identity built on an emperor who had been illegible for two millennia.