Ashoka couldn't capture the South India because he had problems occurring with the Sakas or horsemen of North India. South India was also a dry state which can't provide offers to Ashoka. South India was also conquered by Northern India, Saka, Persians, and Greeks which made it a bit more tough for Ashoka to rebel against the South Indians. As far as I'm concerned, Ashoka never ever fought with the South Indians.
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