Neither Benedict XVI nor John Paul II have been hesitant to assert that they don't approve of the Iraq War, and that they would much rather see peaceful means used to resolve the troubles in that perpetually troubled region. This has led some Catholics in the anti-war camp to start loudly demanding to know when those Catholics who support the war (of which for the record I am one) will make themselves obedient to "the clear teaching of the magisterium on the Iraq War".
I think that one point that is worth making in this regard is as to what can be a magisterial teaching, versus what can be an individual (however well informed and wise the individual may be) prudential judgement. The Church has a set of magisterial teachings regarding the relations between nations and the nature of just war. However, judgements as to a particular war are necessarily not magisterial in and of themselves. They are applications of magisterial teachings.
However, looking at the Vatican's record on the conflicts of the last fifteen years, it looks to me like recent popes' judgements on specific wars primarily stem from a development in their basic assumptions that serve as inputs in making an analysis of whether a war is just. I suspect that this goes back as far as World War I, but since I know more about the events I've been around for, I'd like to particularly take a look at the first Gulf War in 1991.
You would think this would be a pretty basic moral judgement in regards to just war. A larger country with a rather infamous dictator announces that it is going to enforce it's long-standing territorial claims against a much smaller neighbor and invades. The allies of the conquered country first issued ultimatums, then build up military forces, then finally expel the invader. All done in a highly multi-lateral fashion with the blessing of the UN.
However, John Paul II spoke repeatedly against any attempt at a military liberation of Kuwait. I'm not saying that John Paul II should have been going medieval-papacy on the situation: hurling forth excommunications and interdicts and demanding that all able-bodied men take up cross and sword and go forth to right injustice. However, it seems to me that the pope went far beyond simply calling for all possible diplomatic routes to be tried first, and decrying the indiscriminate suffering that has, throughout history, been caused by war. His statements seem strongly to suggest that he believed that war was simply not an acceptable solution to the problem period -- that trying more diplomacy would always be preferable even if months dragged into years and the likelihood of success approached even closer to zero than it already was.
But if it is possible, though force of arms, to expel an invader from a conquered country quickly and decisively, and yet that situation does not meet just war criteria, one is left to wonder: what does? If even expelling an invader is not just, it starts to suggest that no war could ever be just.
Now, I don't actually think that John Paul II (and Benedict XVI following in his footsteps) was a pacifist in the sense that he literally believed that war was in itself never just and could never be morally waged. However, I do think that he held a position which approached de facto pacifism in that (perhaps quite rightly given his life experiences in Eastern Europe in WW2 and the Cold War) he weighed the negative effects even of success in war as being so great that no real world situation was ever likely to justify them in a just war analysis.
Recall that two of the elements in just war morality, as in self defense, are likelihood of success and proportionality. Thus, if one weighs the likelihood of "success" (as in, producing a result any more just than the current situation) as near nil, and the suffering caused in the process as near infinite, then clearly you're never going to find in favor of military action in any given analysis.
Now honestly, I think it's probably a good thing to have our religious leaders holding us back from war rather than urging us on. If our religious leaders were cheerleading military action, who exactly would tell us to think carefully about the suffering about to be unleashed?
And yet, I think that before someone declares the pope to have made the only possible Catholic analysis of the justice of taking military action in a given circumstance, it's important to remember that (based on his experiences living in Eastern or Central Europe during the 20th century) he is using a set of assumptions that makes it virtually impossible to see any military action as justified.