Pope Francis on Monday endorsed military action to stop Islamist militants from attacking religious minorities in Iraq, a rare pronouncement that goes against the Vatican’s usual guidance against the use of force.

Francis cautioned that no country should act alone, and he called for an agreement within the international community, possibly through the United Nations, before embarking on a military campaign. He also warned against an all-out war, insisting that force could be justified only to “stop” the Islamic State, Muslim extremists who have forced Iraqi Christians and members of the Yazidi sect to convert to Islam. Those who refused have fled, and some have been executed by the militants.

James Bretzke, a priest and professor of moral theology at Boston College, said popes in recent history have all lined up against any military intervention, including World War I, World War II, the Vietnam War and, most recently, the U.S. invasion of Iraq in 2003.

“This is the most pronounced endorsement of the use of force of any pope … in the last 100 years,” Bretzke said.

Francis made his comments aboard his papal plane when asked whether he approved of the unilateral U.S. airstrikes on the militants, who have captured key cities in Iraq and Syria.

“In these cases, where there is an unjust aggression, I can only say that it is licit to stop the unjust aggressor,” Francis told reporters as he returned from South Korea, according to the Associated Press. “I underscore the verb ‘stop.’ I’m not saying ‘bomb’ or ‘make war,’ just ‘stop.’ And the means that can be used to stop them must be evaluated.”

Throughout history, he said, 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. “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.”

His comments were striking in comparison to how vigorously recent popes, including Francis, have spoken out against any kind of military intervention.

At the height of the Vietnam War, Paul VI pleaded during a 1965 speech before the United Nations, “No more war, never again war.” As the U.S. prepared for its 2003 invasion of Iraq, John Paul II sent an envoy to Washington and spoke of war as the “very last option.”

Francis spoke out against the use of military force last year. During an August massat St. Peter’s Square that took place two weeks after reports of a chemical weapons attack near Damascus, the pope urged the 100,000 people in attendance to oppose any military action in Syria. “Violence and war are never the way to peace,” he said.

Bretzke said the situation in Iraq is different. He said Francis seemed to be going down the checklist of criteria needed to satisfy the “Just War” theory accepted by the church. The situation appears to be one of last resort, the pope is clearly calling for a “proportional” response and he is calling for a “legitimate authority,” such as the U.N., to make the final call for military action.

“It is fair to say that there is a real change of how these ‘Just War’ criteria are being evaluated and applied in this current situation by this pope,” Bretzke said. “You could go way back in history and things were different. You had popes blessing, quite literally, crusades. You had popes themselves leading forces into battle. But nothing this explicit has come out in the last 100 years.”

Another papal scholar said Francis’ statement, while rare, is not unprecedented in recent decades. Gerard Powers, a professor of Catholic peacebuilding at the University of Notre Dame, said John Paul II made similar comments in the 1990s when genocide was occurring in Bosnia.

In both cases, Powers said, the popes were dealing with situations where groups were being targeted and killed, not broader wars between countries.”It’s not often that popes in recent memory have said that the use of military force was justified, to be sure,” Powers said. “But it certainly fits with what the church has been teaching, and with what popes have been saying about humanitarian intervention situations.”

The U.S. began launching airstrikes against Islamic State fighters on Aug. 8, allowing Kurdish forces to fend off an advance on their regional capital of Irbil and to help tens of thousands of Yazidis escape. President Obama said the primary purpose of the strikes was to protect U.S. personnel based in Irbil.

Since the U.S. attacks, Vatican officials have hinted that they could support those actions. 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.”

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.

Francis also confirmed he hopes 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.