Conclusion. Neither Gratius, nor any other bourgeois scholars of international relations and international law managed to find out the reasons of war and the principle difference between just and unjust war. One of my sources says that only Marxist theory managed it. According to Marxist’ point of view just war is not a predatory one but a war of liberation, which has a goal of protecting the people of external attack or of freeing colonies from the “oppression” of imperialism, etc. And unjust war is a predatory war, which has a goal to conquer and slave the other state’s people.
But I must say that these views are out-of-date of course.