Thus, whether a particular war is just is something Catholics can disagree on in good conscience. As Pope Benedict wrote to the American Bishops when he was head of the CDF:
"Not all moral issues have the same moral weight as abortion and euthanasia. For example, if a Catholic were to be at odds with the Holy Father on the application of capital punishment or on the decision to wage war, he would not for that reason be considered unworthy to present himself to receive Holy Communion. While the Church exhorts civil authorities to seek peace, not war, and to exercise discretion and mercy in imposing punishment on criminals, it may still be permissible to take up arms to repel an aggressor or to have recourse to capital punishment. There may be a legitimate diversity of opinion even among Catholics about waging war and applying the death penalty, but not however with regard to abortion and euthanasia."Importantly, this instruction came after the Iraq war began and in response to requests by American bishops -- thus, clearly, the "war" referred to here includes the Iraq war. So even a Catholic can be "at odds" with the Holy Father on "the decision to wage war" in Iraq and remain in good standing with the Church, while one who supports abortion or euthanasia cannot.
(Worthiness to Receive Holy Communion: General Principles, June 2004)
IOW, a politician's position on war is "negotiable" in a way that his position on abortion is not. It simply does not have the moral weight of an intrinsic evil. The USCCB acknowledges the same point when it says in Forming Consciences for Faithful Citizenship (FC) 33 that although their guidance on prudential issues (including the war in Iraq) are an "essential resource," they "do not carry the same moral authority as statements of universal moral teachings."
The other very important fact to face is that although we may disagree with the decision to have gone to war in Iraq, that decision is now past, and right now our choices are really only:
1) stay until conditions in Iraq are stabilized (McCain), or
2) leave on a fixed schedule regardless of consequences (Obama)
Although the Holy See warned against going to war in Iraq (which I completely agreed with even at the time, btw), later pronouncements which got a lot less press here in America have warned against the second option of a rapid pull-out after Saddam was removed from power. Rather, the Holy See and our Bishops have wanted the coalition forces to bring stability to the country before leaving and to help with the reconstruction. (For example see here and here.) There are hopeful signs that this is happening, such as recently in Anbar province. The stability in the region is much, much greater than it was before the troop surge. I personally feel it would be irresponsible to simply leave -- that would leave a power vacuum which could easily result in anarchy or in another tyrannical regime like Saddam Hussein's. Either way, there would be even greater suffering for the Iraqi people. The Bishops' official position is very similar:
The war in Iraq confronts us with urgent moral choices. We support a “responsible transition” that ends the war in a way that recognizes the continuing threat of fanatical extremism and global terror, minimizes the loss of life, and addresses the humanitarian crisis in Iraq, the refugee crisis in the region, and the need to protect human rights, especially religious freedom. This transition should reallocate resources from war to the urgent needs of the poor. (FC 68)McCain's Iraq strategy is option 1: stay long enough to stabilize conditions in Iraq and ensure the Iraqi government and troops are strong enough to maintain that stability before withdrawing troops. He supported the troop surge, which is now acknowledged even by his opponents as having been effective. Indeed, because of the reduction in violence, President Bush has withdrawn five brigades from Iraq this year. The Iraqi government also supports the McCain plan.
Obama has called for an "immediate withdrawal" with a hard deadline of 16 months, i.e. option 2, which seems at odds with the requirements of a "responsible transition" outlined above. In contrast, a June 2008 article in the Washington Post reported on a telephone conversation Obama had with the Iraqi foreign minister, who "said he was reassured by the candidate's response, which caused him to think that Mr. Obama might not differ all that much from Mr. McCain." However, this July article says that Obama has decided to stick with his 16 month timetable. The fact that he sticks to the schedule irrespective of the actual situation on the ground troubles me -- it seems that the consequences to Iraq are not as important to his plan as the mere keeping of a schedule.
In short, it appears that McCain's Iraq strategy is entirely compatible with what the US Bishops and the Holy See are calling for, and Obama's might be also. But the immediate withdrawal / fixed deadline position is not.
Update: Joe Biden indicated in the recent VP debate that Obama is back to the 16 month arbitrary deadline.