With the Catholic Church, of all things, turning against the doctrine that maintains there can be a "just war," it's worth taking a serious look at the thinking behind this medieval doctrine, originally based in the divine powers of kings, concocted by a saint who actually opposed self-defense but supported slavery and believed killing pagans was good for the pagans -- an anachronistic doctrine that to this day still outlines its key terms in Latin.
Laurie Calhoun's book, War and Delusion: A Critical Examination, casts an honest philosopher's eye on the arguments of the "just war" defenders, taking seriously their every bizarre claim, and carefully explaining how they fall short. Having just found this book, here is my updated list of required reading on war abolition: