Kalinga War

The Kalinga War fought between the Mauryan Empire and the state of Kalinga, an important kingdom that controlled the trade routes with Southeast Asia. Ashoka, the Mauryan king, wanted to capture Kalinga and so his huge army there in 262 BC.
According to Megasthenes, the Greek historian at the court of Chandragupta Maurya, the ruler of Kalinga had a powerful army comprising infantry, cavalry and elephants. After the ear, 150,000 men and women were deported from Kalinga and forced to work in clearing the wasteland for future settlements, and they were forbidden from ever returning to their old homeland. The descendants of the Kalinga people migrated later to Sindh. From there, they migrated to Persia, Mesopotamia, Egypt and Europe. Their descendants might be the Roma of Europe. 
Although he won the war, Ashoka realized the futility of war. He saw the bloodshed and suffering of the soldiers' families and vowed never to fight again. He converted to Buddhism and he dedicated his later life to help his subjects. He stopped conquering after he won a war. He wrote inscriptions for wanting that his son and grandson also never to think about war. He decided to observe Dhamma, a common code of conduct and was also inspired by the teachings of Buddha. He sent many Buddhist scholars to distant lands to propagate Dharma. It is said that one lakh people died in this war. Many pillar and rock edicts were written on the guilt that he felt after seeing the bloodshed.