An emperor's own inscriptions describe the war that made him renounce war

Around 261 BCE, the Mauryan emperor Ashoka conquered Kalinga, and later carved his own account of the cost into rock: 100,000 killed in the fighting, 150,000 deported, and many more dead afterward from the fallout. Overwhelmed by what he'd done, Ashoka renounced further conquest and turned to Buddhist principles of nonviolence, recording his change of heart across edicts inscribed on rocks and pillars throughout his empire.

— Wikipedia contributors, Kalinga War — Primary source: Ashoka's Rock Edict XIII, c. 260s BCE