The recent disagreements between Pope Leo XIV and President Donald Trump have been discussed at length by news reporters, late night comedians, and social media influencers. Some have said that the pope should “stay in his lane” and not delve into global issues like war. However, any pope has both a right and a responsibility to address matters of faith and morals. Morality is more than personal; it is also social. The Holy Father must speak out on issues that have an impact on human beings, including war and peace.

The ministry of any pope or bishop includes a threefold task of teaching, sanctifying, and governing the Church. One very important aspect of the teaching ministry of the Church is called Catholic Social Teaching, which includes basic principles about human rights, justice, the common good, the dignity of human work, the economy, the right to life, the environment, human development, and war and peace.

The Church’s position on war and peace is older and broader than any one head of state or conflict. Our teaching is rooted in Scripture and in 2000 years of lived experience in struggling with the ethics of military action.

A core principle in the Bible that all of us must take seriously is the fifth commandment, “You shall not kill” (Ex 20:13). The people of Israel understood this to prohibit murder, but not the forms of killing that were authorized for them, such as war or capital punishment. The prophets Isaiah and Micah both spoke about a future glorious age in Israel’s future when there would be the fullness of justice and peace: “They shall beat their swords into plowshares, and their spears into pruning hooks; nation shall not lift up sword against nation, neither shall they learn war any more” (Is 2:4 and Mi 4:3).

In contemporary discussions about the justifiability of war, some Christians bring up various passages in the Old Testament that sanctioned military action, portraying a concept of God as authorizing military conquest of other nations. Some examples are Deuteronomy 7, Deuteronomy 20, and 1 Samuel 15:1-9. In their original historical context, these passages give Israel explicit instructions for wars and even the total destruction of certain groups. However, from our perspective as Christians, we interpret the Old Testament in light of the New Testament, based on the fullness of God’s revelation in Jesus Christ.

From a Catholic Christian perspective, those war-and-violence passages from the Old Testament are not to be read as some kind of permanent permission for wars of national domination today. The revelation of God unfolded in successive stages, and the ultimate key for understanding the truth of Scripture is in the fuller light of the Gospel of Jesus Christ (see Benedict XVI, Verbum Domini, no. 42).

Jesus taught the fullness of truth. He reaffirmed the commandment “You shall not kill” in Mark 10:19. In his Sermon on the Mount, he taught “Blessed are the peacemakers” (Mt 5:9). He raised the moral teachings of the Old Testament to a higher standard, saying, “You have heard that it was said to the men of old, ‘You shall not kill, and whoever kills shall be liable to judgment.’ But I say to you that everyone who is angry with his brother shall be liable to judgment” (Mt 5:21-22). He continued, “You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’ But I say to you, love your enemies and pray for those who persecute you” (Mt 5:43-44). At the Last Supper, Jesus said, “Peace I leave with you; my peace I give to you; not as the world gives do I give to you” (Jn 14:27). In the Garden of Gethsemane, he said, “Put your sword back into its sheath, for all who take the sword will perish by the sword” (Mt 26:52).

The earliest Christians were pacifists, refusing to serve in the military for a variety of reasons. For the first 170 years, Christians did not serve in Roman armies. The earliest historical reference to Christian soldiers was in A.D. 173, when some Christians served as soldiers of Marcus Aurelius.

A further development took place when the Roman Empire became officially Christian in the fourth century. With Christians as citizens and in positions of government, they had to deal with the need to protect the state and its innocent citizens from unjust aggression. St. Ambrose, the Bishop of Milan, was the first to formulate a Christian just war theory, building upon the writings of Marcus Tullius Cicero. He placed three conditions that needed to be met for military action to be considered morally acceptable: it should be for a just cause, the conduct of the war must be just, and monks and priests must be noncombatants.

Next, St. Augustine, the Bishop of Hippo, developed just war theory into five parts. For military action to be just, the following conditions must be present:

1. The cause must be just.
2. The conduct of the war must be just.
3. Those fighting must have a just disposition.
4. It must be carried out under the authority of the legitimate ruler.
5. There are some people who are exempt: clergy and civilians.

In the 13th century, St. Thomas Aquinas clarified the principles of just war doctrine, adding these conditions:

1. War should be undertaken only as a last resort.
2. It should have a probability of success.
3. There should be proportionality between the damage incurred and the end pursued. 

Over the centuries, the just war doctrine of the Church has continued to be clarified. It gives strict conditions for the justification of military action. There is always a strong presumption against the use of lethal force. A war of aggression is intrinsically immoral (Compendium of the Social Doctrine of the Church, no. 500). For war to be considered morally justified, all the following criteria, not just some, must be met simultaneously:

1. Just Cause: War must be necessary to deter or repel unjust aggression. Some serious violation of human rights needs to be stopped. The damage inflicted by the aggressor must be lasting, grave, and certain.
2. Competent Authority: The use of force must be authorized by lawful public authorities.
3. Right Intention: Such as protection of human rights or protection against real or threatened injury, not for vengeance or exploitation.
4. Last Resort: Only when all peaceful alternatives to deter or reverse the aggression have been exhausted.
5. Probability of Success: Troops should not be thrown into a hopeless massacre.
6. Proportionality: The use of arms must not produce harm and disorders that are graver than the harm that one is seeking to overcome.
7. Noncombatant Immunity: Civilians must not be targeted (see Compendium of the Social Doctrine of the Church, no. 500; Catechism of the Catholic Church, no. 2309).

The principles of just war doctrine are violated by terrorism, by weapons of mass destruction (including nuclear weapons), by the deliberate targeting of civilian infrastructure, and by the intent to annihilate an entire civilization.

According to the Catechism of the Catholic Church, no. 2309, it is public authorities, not religious leaders, who must make the call about whether the just war conditions are satisfied in a particular case. A leader of government should take the general principles of the just war doctrine and apply them to the particular situation, making a prudential judgment of conscience.

What this means is that different people of good will can assess the same situation, according to the information available to them, applying the same general principles, and they can arrive at different judgments of conscience about a war. The assessment of the conditions of a just war belongs to the prudential judgment of those who have responsibility for the common good. Ultimately, our elected leaders are responsible for decisions about national security (CCC, no. 2310).

Now that our country has undertaken significant military action in Iran, I recommend that we all pray and hope for the following:

1. that this conflict will end soon;
2. that innocent civilian life will be protected;
3. that weapons of mass destruction will be eliminated;
4. that the people of Iran and their neighbors will soon enjoy peace with freedom and justice;
5. for Pope Leo XIV and those who assist him in his ministry;
6. for all government officials, in the United States, Iran, and all countries, that they may exercise their duties with justice, wisdom, and good judgment;
7. for the men and women who risk their lives in the service of our nation in the military, for their families and loved ones. We believe that when soldiers carry out their duty with honor, they contribute to the security and freedom of nations and to the maintenance of peace (see Gaudium et Spes, no. 79);
8. for the people of Iran and other countries who have suffered for so long under brutal regimes;
9. that all terrorists will experience a change of heart and respect the God-given dignity of every human being;
10. for those who work to provide for the humanitarian needs of all those impacted by war.

“Blessed are the peacemakers, for they will be called children of God” (Mt 5:9).