The conquest
Kalinga lay on the east coast, roughly modern Odisha. Ashoka's thirteenth Major Rock Edict says it was conquered eight years after his consecration, and gives the toll in plain numbers: a hundred and fifty thousand people carried away, a hundred thousand killed, and many times that number dead.
Remorse
The edict goes on in words unique in the ancient world. The Beloved of the Gods, it says, feels deep remorse for the conquest, because the killing, death and deportation of people are grievous to him. Even the suffering of good people caught up in war, and of their friends and relatives, weighs on him.
From then on, he declares, the best conquest is conquest by dhamma, by right conduct, and he hopes his sons and grandsons will not seek new conquests by arms.
A change of policy, not an end to power
Ashoka did not disband his army or give up Kalinga. The same edict warns the forest peoples that the emperor has power as well as mercy. Tellingly, this edict was not set up in Kalinga itself. There, two special edicts address local officials instead, urging them to treat the people fairly.
Why it matters
Ashoka increasingly promotes an ethical policy of dhamma and becomes Buddhism's most famous early royal patron.
History and tradition
What historians can say
The Kalinga war is known from Ashoka's own account. That he chose to publish his remorse is itself remarkable. How far the war caused his turn to Buddhism is debated: the edicts suggest he had already become a lay follower around this time.
What Buddhist tradition remembers
Buddhist legend often tells the story of his conversion differently, through a meeting with a young monk, without mentioning Kalinga at all.