In this article, the writer wants to show the teachings of Thucydides, Greek historian from old times, regarding the violent phenomenon of civil war. With war as a framework and human nature as underlying background, Thucydides notices the violent, thoughtless, and vengeful behavior of human beings; the evil found in the use of the lenguage and its influence on human performance; the moral degradation clearly visible in evil, fanaticism and corrupt political activity; the success of slyness and recklessness over kindness and strength; and, as a concluding idea, the consciousness of the dangerous and self-destructive nature of rational beings when they lose control of themselves and disregard human and divine laws.