ROME -– Pope Francis delivered a mixed verdict on US airstrikes in Iraq on Monday, saying that while it’s morally legitimate to stop an unjust aggressor, as America says it’s doing with regard to the radical Islamic State in northern Iraq, a single nation shouldn’t decide for itself when the use of force is warranted.

In light of the Vatican’s recent history of vigorous opposition to virtually any American intervention in the Middle East, the pope’s language is likely to strike observers as a cautious yellow light.

The pontiff revealed that he has considered traveling to Iraq in the near future to demonstrate his concern over the crisis, including its impact on the country’s Christians, and that doing so remains an option.

For now, however, Francis said, such a journey is “not the best thing to do.”

As he has already done indirectly, Francis confirmed his intention to travel to Philadelphia in September 2015 for a Vatican-sponsored meeting on the family. Since he’s also been invited to Washington to speak to Congress and New York to address the United Nations, he said, “Maybe [the trip] will be the three cities together.”

Despite having been forced earlier in the summer to cancel several appointments due to illness and exhaustion, Francis played down concerns about his health. The 77-year-old pontiff conceded, however, that he may need to be “a little more prudent” in protecting his energy.

The comments came during an airborne press conference on the return flight from Francis’ Aug. 13-18 trip to South Korea. The pope spoke to reporters for a full hour, taking 15 questions on a wide range of subjects.

Asked whether he approved of the US bombing campaign in Iraq, Francis said that “I can say only it’s licit to stop an unjust aggressor.”

The pontiff clearly expressed doubts about unilateral action.

“A single nation cannot judge how to stop an unjust aggressor,” he said.

The United Nations, according to Francis, “is where it must be discussed and asked, ‘Is there an unjust aggression?’” and, if so, ‘How do we stop it?’”

The pope also said the world needs to remember past instances in which strong nations have claimed to be protecting minority groups, but in reality have used force to advance their own agenda.

“How many times, under the excuse of stopping an unjust aggressor, have powers taken over peoples and launched a true war of conquest?” the pope asked.

While Iraq’s Christian minority has been among the primary targets of the Islamic State, Francis insisted it isn’t just Christians who are at risk.

“There are men and women, religious minorities, not just Christians, and they’re all equal before God,” he said.

The position on the US action outlined by Francis does not mark a substantive departure from the Vatican’s usual line on military force, since Rome has long insisted that for any intervention to be legitimate it must have an international warrant.

The comments were more restrained, however, than those of senior Vatican officials who recently seemed to endorse the US strikes. The pope’s ambassador in Iraq has described the strikes as “something that had to be done,” while the Vatican’s envoy to the U.N. in Geneva called them “probably necessary.”

In terms of a possible trip to Iraq, Francis said “I think I can tell you” that he’s discussed it with his aides.

“We said that maybe if it’s necessary, when we return from Korea we can go to Iraq,” the pope said, marking the first acknowledgement that such a trip was under consideration.

“I’m willing to go,” Francis said. “At the moment it’s not the best thing to do, but I’m open to it.”

On other matters, Francis said that he would travel to China “tomorrow” if the opportunity presented itself.

China is among a handful of nations that does not currently have diplomatic relations with the Vatican, and the country’s roughly 13 million Catholics are often subject to harassment and persecution. The Vatican has made improving relations with Beijing a top priority.

Some saw a potential thaw last week when Francis became the first pope to fly over Chinese airspace en route to South Korea on Aug. 13, and he dispatched a friendly telegram invoking “divine blessings” on the nation.

Francis said the church seeks “only the liberty to do its duty and its work” in China, imposing “no other condition,” and that the Vatican is “always open” to improved ties.

In terms of future travel, Francis said that trips to Mexico and Spain are under consideration for 2015 but cannot yet be confirmed.

Asked how he deals with his massive popularity, Francis said that he tries to focus on “my own sins and my own mistakes,” as well as reminding himself that in light of his advanced age, “It won’t last.”

Francis confirmed that he’s working on an encyclical, the most developed form of papal teaching, on the environment. He said a first draft has been submitted for his consideration, but he wants to cut it down in order to distinguish “certainties” of the Catholic faith from scientific theories, “some of which may be fairly certain and others less so.”

Finally, the pontiff offered a vigorous defense of his peace initiative following a May trip to the Middle East, when he invited the Israeli and Palestinian presidents to join him for a peace prayer in the Vatican gardens on June 8.

That peace prayer was followed just days later by an Israeli offensive on the Gaza Strip and a new clash between Israel and Hamas, and the pope was asked if his efforts had failed.

“It was absolutely not a failure,” Francis insisted.

The pope said both President Mahmoud Abbas of the Palestinian Territories and former President Shimon Peres of Israel, the leaders who joined him and Bartholomew I, the Orthodox Patriarch of Constantinople, for the prayer, are genuine “men of peace.”

“Under the smoke and the bombs right now, it’s hard to see the door that was opened,” Francis said, “but that door is still open.”