Ashoka attacked Kalinga, leading to his profound remorse.

Evidence

When he became emperor he set out to complete the occupation of the whole subcontinent and attacked the independent state of Kalinga – modern-day Orissa on the east coast. It was a savage, brutal assault and one which seems afterwards to have thrown Ashoka into a state of terrible remorse. — Neil MacGregor, A History of the World in 100 Objects, 532

Categorization

• corroborated: 2 independent passages support this claim — Neil MacGregor, A History of the World in 100 Objects, 532