Just war theory deals with the justification of how and why wars have to fight. The justification can be either theoretical or historical. the “just war tradition,” The just war tradition may also consider the thoughts of various philosophers and lawyers through the ages and examine both their philosophical visions of war’s ethical limits (or absence of) and whether their thoughts have contributed to the body of conventions that have evolved to guide war and warfare.
Article 51 of the UN Charter clearly recognizes “the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations” by anyone. That is, the language of Article 51 does not identify or stipulate the kind of aggressor or aggressors against whom this right of self-defence can be exercised … and certainly does not limit the right to self-defence to attacks by State only!