WELLESLEY -- Father Bryan Hehir, secretary of health and social services for the Archdiocese of Boston, has been teaching about religion, ethics, and foreign policy for 40 years. He is fond of giving his students a challenging moral dilemma.

He asks them to imagine themselves as a military leader during a conflict. An enemy tank factory has been discovered -- but it is located next to a kindergarten, a school, and a hospital.

"We'll do our best," a commanding officer explains, "but we can't guarantee that none of them would be killed."

Would you approve an attack? Must civilian casualties be kept to zero at all costs? Or should the enemy target be neutralized by any means necessary? How many deaths is an appropriate price to pay for victory?

Father Hehir attempted to answer these difficult questions from a Catholic perspective during "Two Wars: One Moral Framework," a talk about the morality of the current conflicts in Ukraine and the Holy Land. Father Hehir delivered the talk to a packed house at St. John the Evangelist Parish in Wellesley on Jan. 25.

In his talk, he said that Russia's invasion of Ukraine in February 2022 is neither legally or morally justifiable according to Catholic teaching on warfare.

"As a sovereign state, Russia has the right to use force," he said, "but only under certain circumstances, and those circumstances were violated by aggression."

He also condemned Hamas's attack on Israel on Oct. 7, 2023, but added that Israel must do more to avoid civilian deaths in Gaza. He does not believe that Israel is deliberately targeting civilians, but that its military must plan more carefully to prevent their deaths. The war in the Holy Land is an example of urban warfare, he said, which puts many more civilians at risk.

"The right to self-defense is not an absolute right," he said. "It must be done within limits."

According to Catholic moral teaching, he said, the use of force must be "proportional to the threat" and done with a reasonable expectation that the war can be won. He also decried the spike in antisemitism and anti-Arab sentiment that followed the outbreak of war in the Holy Land.

Father Hehir opened his talk by describing the three primary schools of thought on the morality of war: pacifism, classical realism, and the just war theory.

"The central standard is that it is not permissible to justify the purposeful taking of a human life," he said of pacifism. "And so, pacifism as an absolute rule, it becomes almost a way of life, because it is not an easy position to hold to, especially when conflict begins, and passions run high."

He described classical realism as "a position that when wars are fought, interests are at stake, but there is hardly any room for moral restraint on those interests."

Classical realism has existed in western civilization since ancient times. Father Hehir said that its philosophy can be best summed up in the words of the ancient Greek historian Thucydides: "The strong do what they can, the weak suffer what they must."

"This is not a religiously-ground position," Father Hehir said. "It is a philosophical position, a political position."

This view is "widely held" in society and government, Father Hehir said, especially when conflicts begin. However, it is not completely amoral and without restraint. Morality is "an aspiration," he said, "but not a restraining force."

In between these two positions, he explained, is the just war theory that holds that the use of force is morally acceptable in cases such as national self-defense.

"This position has been the dominating position of the Catholic Church since the fifth century," Father Hehir said. "But as with so many things today, Pope Francis has his own way of addressing these questions."

He described Pope Francis's position as being somewhere in between pacifism and just war theory.

Father Hehir said that the wars in Ukraine and the Holy Land have many similarities. Both are fought by neighboring countries with deep roots, long and fraught histories, and a litany of grievances on both sides. Both began with invasion, but while the invasion of Ukraine was perpetrated by a world power, the Oct. 7 attack on Israel was perpetrated by a terrorist group not recognized as a legitimate government by the rest of the world.

"In each case, the invasion constituted an act of aggression, both in moral and legal terms," he said.

Both are conventional conflicts, like the World Wars, but fought with 21st-century technology such as drones.

"The shadow of nuclear weapons stands behind both," he said. "In these two conflicts, nuclear weapons are not up front. They are not likely to be used, but they could be used."

Also like the World Wars, he said that the world's countries are each taking sides in the conflicts. He described the two wars as taking place under the "shadow" of "great power politics." He explained that, like the European empires that exerted global influence in the 19th century, the three nuclear superpowers of Russia, China, and the U.S. dominate today's global affairs.

"It's hard enough to manage the nuclear age when there are two powers that look each other in the eye and see the shadow of nuclear weapons," Father Hehir said, referring to the Cold War between the U.S. and the Soviet Union.

The rise of China makes things "much more complicated."

"The question of who is deterring who is much harder to figure out," he said.

Surrounding these three superpowers are lesser powers like Israel, Iran, South Africa, and India, which have their own nuclear arsenals. Surrounding them are the rest of the world's countries, as well as international organizations and institutions. Father Hehir said that as more and more countries choose sides in the two wars, they have the potential to expand across their respective regions.

"These wars are what we would call an existential conflict," he said. "An existential conflict means that to a state or people, if they lose this war, they lose their identity."

Therefore, nations are desperate to defend themselves, but Father Hehir warned that such defense must be done ethically.