The Ethics of War : shared problems in different traditions by 
The Ethics of Preventive War by 
Morality and War : can war be just in the twenty-first century? by 
The Justification of War and International Order by Subject search terms to find more books of interest on SOLO
War -- Moral and ethical aspects
Just war doctrine
War (Philosophy)
SOLO searches will probably reveal that other parts of the Bodleian such as the Social Science Library, the Philosophy and Theology Faculty Library, and the History Faculty Libraries hold useful titles.
Please check their opening hours on their official websites below
The collection in the Law Library tends to be stronger on the practical results: the rules or agreements that have applied in various wars across the ages or have been constructed in the hope of limiting future conflicts