The issue of war has always been one of the hottest topics in philosophy, and it has been written about in the Western philosophical circle since antiquity. Among the ancient thinkers in this work, the view of Thucydides, who wrote about the Peloponnesian War, which was later followed up by Thomas Hobbes in his Leviathan, stands out. Michael Walzer presents and attacks their arguments in order to deal with realism and moral relativism, and then sets up a modern theory of just war, which is based on two principles, Jus ad Bellum, i.e. the justice of war, and Jus in Bello, i.e. the just conduct of war . After establishing these principles, the paper focuses on the categorization of different types of war according to their justice, starting with aggression, then preventive war, pre-emptive war, military intervention, guerrilla war and terrorism. Individual cases belonging to these types of war are discussed and compared with the principles of just war that have been set. In the part that deals with the end of the war, the importance of its timely and just end is stated. After that, I present Walzer's concept of a war convention, with which he tries to determine the duties in war of conflicting states, war commanders and ordinary soldiers. Although arguments are also presented, primarily criticism from other philosophers, the paper primarily presents the theory of just war as conceived by Michael Walzer, which has a central place in the contemporary ethical debate on war for the last 40 years and more.