In August, John L. Allen, Jr., wrote in the Boston Globe that for “anyone familiar with the Vatican’s recent history of bitter opposition to any US use of military force in the Middle East, Rome’s increasingly vocal support for the recent American airstrikes in Iraq may seem, to say the least, a little disorienting.”  While it is to be cautioned that the Holy Father has not called for military action against the Islamic State, the disorientation that Mr. Allen refers to might be ameliorated somewhat by a review of the Catholic Church’s Just War Doctrine. According to the Catechism of the Catholic Church, “defense by military force” (military aggression is never justified) must meet all of four conditions to be legitimate:
“- the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain;
“- all other means of putting an end to it must have been shown to be impractical or ineffective;
“- there must be serious prospects of success;
“- the use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modern means of destruction weighs very heavily in evaluating this condition.” 
There can be no serious argument that the first three conditions are not met with regard to the actions of the Islamic State. It has engaged in unprovoked conquest of the territory of two sovereign nations, has subjected many in those conquered lands to harsh treatment, engaging in well-known human rights abuses, and has stated its intention to establish its authority over all Muslim inhabited lands. While it would be ridiculous to contend that it could somehow be talked out of continuing in this fashion, there can be little doubt that a coalition that includes the United States and other Western powers could be expected to be successful in dislodging the Islamic State from the territory it has conquered.
It is the fourth condition that is of concern, and whether or not that condition will be met is largely in the hands of the United States. True, it would take some doing to “produce evils and disorders” more serious than war crimes  and ethnic cleansing.  But these atrocities should not be replaced by continuing civilian casualties as the U.S. restricts its operations to attacks from the air. In the end, whether the United States once again runs afoul of the Just War Doctrine in its actions against the Islamic State will depend entirely on the United States.