At NRO , George Weigel has a brief but enlightening article on the just war tradition :
The classic just-war tradition did not begin with a presumption against war. Augustine didnt begin there; Aquinas didnt begin there. And indeed, no one in the tradition began there until the late 1960s (surprise!), when a Congregationalist moral theologian (James Gustafson) sold a Quaker moral theologian (James Childress) the idea that the just-war way of thinking began with a prima facie moral duty to do no harm. Childress then successfully sold the notion to J. Bryan Hehir, the Catholic theologian and political theorist who was the chief architect of The Challenge of Peace.
In fact, however, the classic just-war tradition began, not with a presumption against war, but with a passion for justice: The just prince is obliged to secure the tranquility of order, or peace, for those for whom he accepts political responsibility, and that peace, to repeat, is composed of justice, security, and freedom.
And at The Volokh Conspiracy, Kenneth Anderson adds more insightful commentary on just war, international law, and Catholicism:
And as for the Vatican well, I am not a Catholic or Catholic theologian, but in following Vatican statements concerning the use of force, I have long been struck that the Vatican does not follow just war ethics as even the formal apparatus of analysis. Summarizing roughly, it seems to follow more closely the European line about the primacy of international law, or anyway a certain, thoroughly unrealistic, but literal, reading of the Charter. I have sometimes wondered if the Vaticans refusal even to speak the formal language of just war ethics the five or seven standard criteria was not intended as a very long term message that, although Americans associate just war ethics with Catholicism, it is not the law of the Church, but only one tradition within it concerning the use of force. I believe Weigel would concur in the observation that the Vatican has abstained from signing onto just war ethics as the formal apparatus for analyzing resort to war.