Here is an article from the Catholic Encyclopedia that comments on these matters :
http://www.newadvent.org/cathen/15108a.htmHere is another one that is a corollary :
http://www.newadvent.org/cathen/15546c.htmThey are interesting and shed some light on the issue. Anyway, something that is often obscure to people today is that the right of war, the authority to defend oneself, and the authority of temporal sovereignties to punish criminals with death all stem from the same natural law conceptual framework. That is to say, in war, one side (or neither) is restoring the damaged peace in the world, but that the concept of a just war derives from the presumption of something lawfully established being attacked or harmed by an unjust aggressor. Anything truly constituted with real authority must also have the authority to defend its existence with coercion, otherwise it only exists in the mind rather than in actuality. Thus, a man can defend himself to the death without sin, and it is in fact virtuous to so defend his family (though in the early Church there was still a penance for shedding blood). Likewise, a nation -- or rather a sovereign prince -- fights to protect the peace of the natural order, of which his own authority participates and thus to which he is beholden.
As an aside, this is one of the reasons that the competition between the nations caused by Protestantism and the rise of towns and moneylending in the Late Middle Ages and Renaissance was so serious : Men began to quarrel to the death in order to protect their own interests at the expense of Christendom, often in defense of their codified vernacular cultures. A single hegemon, a great captain who defends the Church and maintains a single center of gravity that balances the global order and also thereby guarantees the customs of each particular people (not absolutely, but reasonably) within a greater framework -- this would end this endless cycle of violence and ambitious development between the nations. (Even so, of course a true and lasting peace could only be won through the integral reign of the Sacred Heart of Jesus and the Immaculate Heart of Mary.) Many nations sought to recapture the mantle of the Roman Empire with this frame of thought in their minds, each seeking to become the so-called "universal monarchy" upon which the tranquility of Christendom depended (perhaps hindsight is twenty-twenty here, but in any case many nations did indeed seek to become a universal monarchy and to restore the Roman Empire). If Louis XIV had only consecrated himself to the Sacred Heart, this glorious age would have already occurred and perhaps it would still persist.
Anyway, the Church is no stranger to high politics and the causes and realities of warfare. The Protestants are scandalised by this because they imagine that Christianity should be entirely spiritual, having no relation to material things, especially hierarchies and institutions staffed by sinning, mortal men. On the contrary, Our Lord is incarnate as His Church is. Thus, the Christian project for the salvation of the souls of individuals cannot help but to likewise be a project for the security of the common good of all men under the universal reign of Christ. That is to say, Christianity's uniquely sacramental, incarnate vision of the universe is very political (being that "politics" is synonymous with the social life of a temporal community wherein and over authority is wielded by one designated to do so). But the Church does not only exist in the world of abstract principles. It's head is in a real city that was chosen during a real historical time period according to real and specific historical conditions. This state of affairs has, of course, persisted and could not be otherwise. Thus, defending the Christian powers that have a relationship with the Church -- defending the real and existing Christian moral personalities that have been and cannot help but continue to be significant historical actors in the narrative of salvation history -- is plainly a natural duty for the everyday Catholic layman. He is not being tainted by preserving these goods pursuant the rights of Our Lord's Sacred Royalty. On the contrary, he is just being a good Catholic like any other since Pentecost, and that according to the same principles and sense as his forefathers in the Faith.
That is my best foray into an explanation of these matters at the moment.