When Mark Shand and Tara the elephant approached the Daya River in the Indian state of Orissa, black clouds circled overhead. Tara “moved forward reluctantly, her trunk in the air, sensing, probing,” Shand writes in his book Travels on My Elephant. Suddenly, Tara roared, refusing to go any farther. They’d reached the site of an ancient battle in which massive numbers of war elephants were killed, and more than 2,000 years later, Shand believed, Tara could still sense the carnage.

The battle that took place in 261 B.C. by the Daya River and nearby Dhauli hills came to be known as the Kalinga War. The Indian emperor on the battlefield — the man responsible for the slaughter — was Ashoka Maurya, and the massacre resulting from his campaign to join Kalinga to the rest of the Mauryan Empire would cause the once sadistic ruler — according to accounts in the Ashokavadana, a text of intertwined facts and legends — to rethink his ways.

Historical accounts put casualties on both the Mauryan and Kalingan sides of the conflict in the hundreds of thousands, although Kalinga, a much smaller state, suffered more. Sonam Kachru, a philosophy of religions professor at the University of Virginia, says that while it’s impossible to know precisely how many died, it’s probable the “numbers aren’t empirical” but help illustrate that a substantial portion of Kalinga’s male population perished.

Ashoka Maurya — also known as Ashoka the Great, the Blessed One and King Piyadasi — boasted an impressive lineage. His grandfather Chandragupta, founder of the Mauryan Empire, unified great swaths of the subcontinent after Alexander the Great’s influence began to wane. Bindusara, Ashoka’s father, extended the territory of the Mauryas even farther, and as one of his youngest sons, Ashoka was consecrated the Mauryan emperor in 269 B.C. Charles Allen, author of Ashoka: The Search for India’s Lost Emperor, says the ambitious ruler wanted to “pick up from his father” and finish the “conquest of the south.”

But after seeing the horrors of the Kalinga War — the Daya River ran red with blood — Ashoka abruptly changed the fundamental nature of his empire, vowing never again to conquer through violence. While legend and history have blurred over time, Buddhist concepts of compassion and tolerance clearly appealed to the newly repentant Ashoka. He became a follower and patron of Buddhism, which up to then hadn’t taken hold in India.

Around 260 B.C., the Blessed One began erecting stone edicts across his vast domain (present-day Afghanistan, India, Nepal and Pakistan). These detailed his reforms and the principles by which a compassionate state and moral human beings should abide. In addition to spreading the dhamma (Buddhist truths), the edicts were a way for Ashoka to let people know they no longer needed to fear him. “Ashoka offered his subjects a psychological self-portrait, offering them his reasons and emotions … a fleshed-out [spiritual] biography of him as a person,” Kachru explains. No one else in the ancient world had ever done so, which is perhaps why Buddhism began to flourish.

Although Ashoka regretted the Kalinga War, he never gave the small state back, nor did he erect the 13th Rock Edict — in which he expresses remorse for the rampant destruction — in Kalinga itself. Perhaps he didn’t want to open fresh wounds, as he still had an empire to govern, and only his minor edicts mention Buddhism by name (although the dhamma is often cited).

Ashoka sent Buddhist missionaries out into the world, as far as Egypt and possibly Greece to the west, Sri Lanka in the south and China to the east, which means he’s largely responsible for transforming Buddhism from a local heterodox religion into a global one. With an army of missionaries and official state sponsorship bolstering it, Buddhism spread across well-trodden trade routes, where it was met with acceptance, curiosity and suspicion — sometimes even mixing with local traditions, like the eventual amalgam of the indigenous Shinto and Buddhism in Japan.

Brahmanical orthodoxy (Hinduism’s antecedent) and resistance to India’s shift toward Buddhism, as well as invading armies practicing different religions, led to the nontheistic religion’s gradual decline there. Centuries later, Chinese monks on a pilgrimage to India took note of the religion’s disappearance from the land of its birth.

Ashoka, whose impact as a historical figure was suppressed in non-Buddhist circles, has more recently been “rediscovered” and admired by intellectuals. Jawaharlal Nehru, India’s first prime minister after independence, praised Ashoka as an example of a ruler who unified India before the British, and as a man who, according to Kachru, “promoted a kind of morality that cuts across religious traditions.”

Carl Pettit is a journalist and writer, with interests ranging from international relations and political humor to the arts and modern media. Education and work have taken him all over the world, infusing him with a strong case of wanderlust, which he gave up trying to cure ages ago.