Where to find specialist books on this topic.
In the Law Library, these will be on Level 3, one floor above the entrance level to the law library. The floor is accessible via both stairs and lift.
In the public international law collection, browsing these shelf marks will be particularly fruitful
War & armed conflict Internat 750
Termination of war Internat 760
War crimes, trials etc Internat 580
Some works on the definition of war/armed conflict in international law which may be useful starting points are:
International Law and the Classification of Conflicts by 
The Law of Non-International Armed Conflict by 
What Is War? : an investigation in the wake of 9/11 by 
The Ethics of War : shared problems in different traditions by 
Islamic Jihadism and the Laws of War by Article 2(4) UN Charter
"All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations."
Finding books in the Bodleian
Subject search terms which may be useful include:
Aggression (International law)
Intervention (International law)
Just war doctrine
Self-defense (International law)
Recourse to Force : state action against threats and armed attacks by 
The International Court of Justice and Self-Defence in International Law by 
The Ethics of Preventive War by 
The Oxford Handbook of International Law in Armed Conflict by 
The Conduct of Hostilities under the Law of International Armed Conflict by 
Hague Law Interpreted The Conduct of Hostilities under the Law of Armed Conflict by 
Wars of Law unintended consequences in the regulation of armed conflict by 
Weapons and the Law of Armed Conflict by These are listed alphabetically by title.