Letters 95.31

Deeds that would be punished by loss of life when committed in secret, are praised by us because uniformed generals have carried them out. Man, naturally the gentlest class of being, is not ashamed to revel in the blood of others, to wage war, and to entrust the waging of war to his sons, when even dumb beasts and wild beasts keep the peace with one another.

Seneca, Letters to Lucilius 95.31, translated by Richard Mott Gummere