When Ashoka became the king, Kalinga was the only kingdom that was not under Mauryan control. Ashoka attacked Kalinga and conquered it after fighting a tough war.
But this battle proved to be a turning point in Ashoka’s life. He was saddened by the death and suffering caused by the war. He realised that war was a futile affair that only led to death and sadness.
After the war, Ashoka gave up his policy of conquest through war (dig-vijaya) and began to follow a policy of conquest through dharma (dharma-vijaya). The spread of dharma became the goal of Ashoka’s life.