Francis also said he and his advisers were considering whether he might go to northern Iraq himself to show solidarity with persecuted Christians. But he said he was holding off for now on a decision.
The pope's comments were significant because the Vatican has vehemently opposed any military intervention in recent years. Pope Paul VI famously uttered the words "war never again, never again war" at the United Nations in 1965 as the Vietnam War raged, a refrain that has been repeated by every pope since. St. John Paul II actively tried to head off the Iraq war on the grounds that a "preventive" war couldn't be justified. He repeatedly called for negotiations to resolve the crisis over Iraq's invasion of Kuwait a decade prior.
Francis himself staged a global prayer and fast for peace when the U.S. was threatening airstrikes on Syria last year.
But in the aftermath of the Sept. 11 attacks — in the Vatican's mind an "unjust aggression" — John Paul defended the "legitimate fight against terrorism" and the right of nations to defend themselves against terrorist attacks. He did, though, call for restraint and the Vatican subsequently focused its position on emphasizing the need to eradicate the root causes of terrorism: poverty and oppression.
Recently, the Vatican has been increasingly showing support for military intervention in Iraq, given that Christians are being directly targeted because of their faith and that Christian communities, which have existed for 2,000 years, have been emptied as a result of the extremists' onslaught.
The U.S. began launching airstrikes against Islamic State fighters Aug. 8, allowing Kurdish forces to fend off an advance on their regional capital of Irbil and to help tens of thousands of religious minorities escape.
When the Vatican's ambassador to Iraq, Monsignor Giorgio Lingua, was asked about the U.S. airstrikes, he told Vatican Radio that it was unfortunate that the situation had gotten to this point "but it's good when you're able to at the very least remove weapons from these people who have no scruples."
The Vatican's ambassador to the United Nations in Geneva, Archbishop Silvano Tomasi, went further, saying "Maybe military action is necessary at this moment."
Church teaching allows for "just wars," when military force can be morally justified under certain circumstances. The four main criteria, all of which must be met, include that the damage inflicted by the aggressor must be "lasting, grave and certain," that all other means haven't worked, that there must be real prospects for success and that the intervention must not produce results that are worse than the original evil. Finally, church teaching holds that the responsibility for determining if the four conditions have been met rests with the judgment of "those who have responsibility for the common good."
Francis was thus essentially applying church teaching on the "just war" doctrine to the Iraq situation.
But, he said, in history, such "excuses" to stop an unjust aggression have been used by world powers to justify a "war of conquest" in which an entire people have been taken over.
"One nation alone cannot judge how you stop this, how you stop an unjust aggressor," he said, apparently referring to the United States. "After World War II, the idea of the United Nations came about: It's there that you must discuss 'Is there an unjust aggression? It seems so. How should we stop it?' Just this. Nothing more."
Francis sent a personal envoy, Cardinal Fernando Filoni, to northern Iraq last week with an undisclosed amount of money to help people in flight and show the pope's solidarity with those forced to flee their homes.
In other comments Monday:
• Francis confirmed he hoped to travel to the United States in September 2015 for a possible three-city tour: to attend a family rally in Philadelphia and to address Congress in Washington and the United Nations in New York. He said a Mexico stop on that trip was possible but not decided yet. He also said he might make one-day visit to Spain next year.