Speaking of being careful when you're talking about theology with the Pope:
The modern Catholic approach to just war is not a fixed theological position simply derived from Augustine and Aquinas. Strictly speaking, there is no single official 'doctrine of the just war.'
What exists is a doctrinal framework that derives from the Gospels and takes Augustine and Aquinas into account, but whose modern cornerstone is John XXIII's 1963 encyclical "Pacem in Terris" (Peace on Earth).
"Pacem in Terris" is arguably the most important modern encyclical, alongside Leo XIII's "Rerum Novarum" and John's own "Mater et Magistra", and one of the finest ever written.
Produced shortly after the Cuban Missile Crisis, it argues that the terrifying growth in lethality and the potential for indiscriminate destruction by modern weapons renders the traditional concept of 'just war', limited, strictly defensive, responding to grave unjustified aggression, nearly impossible to satisfy in practice.
Without denying the right to self-defense in extreme cases, John XXIII established that outside truth, justice, charity, and liberty, all armed conflict is essentially wrong.
As the encyclical opens: 'Peace on Earth [...] cannot be firmly established unless the order which God has laid down is dutifully observed.'
Peace, in other words, is achievable only through conversion and respect for universal order and universal human rights.
Most of those universal rights, the right to life, to peace, to health, to migration, were not "invented" by John XXIII but derived from earlier Doctors of the Church, including Augustine. Two of them, the rights of conscience and women's rights, are genuinely new as far as the Church position of that time.
War is not absolutely excluded, but it remains a last resort, legitimate only in the rarest and most extreme cases, and always understood as a rupture of God's natural order."
The teachings of "Pacem in Terris' have been upheld by every subsequent Pope without exception, this is not a Francis or Leo innovation.
What changed under Francis, and continues under Leo, is the explicit acknowledgment that any war can be manipulated into appearing 'just.' The marginal cases still theoretically permissible under PiT therefore require even more stringent criteria.
Cardinal Parolin formally called for this revision in 2024. Leo XIV is clearly following suit.
nytimes.com/2026/04/14/us/po…