This article analyzes the evolution of the concept of "just war" throughout the ages by means of a qualitative analysis based on a documentary review. It starts with the study of the notion of just war in Rome, specifically in the work of Cicero. It subsequently delves into the evolution of the concept in the Roman and Medieval Christianism and in the Islamic religion. The article then examines the development of the concept in the Modern Age, carried out by the neo-academics of the School of Salamanca and Hugo Grotius, regarded by many as the first theoretician of international law. After, the article delves into ideas of the legitimacy of war found in the work of Hegel, and then studies the concept of just war in light of Marxism-Leninism, through the works of Marx, Lenin and Mao Tse-Tung. In the twentieth century, the article sets out the different visions on war of the great public law jurists Hans Kelsen and Carl Schmitt: the former supported the need to avoid war through law, while the latter defended its inevitability. Finally, the article analyzes the current regulation of the use of force within the United Nations framework and concludes with a critical appraisal of the issue.
