Nothing familiar to the medieval Christian mind, however, could have prepared it to envision the horrors of the late modern period. We live with the shameful spectacle of tyrants liable to turn against their own people in murderous fury at any time, and of governments helpless to stop domestic genocide and persecution when they are not themselves, as they often are, active perpetrators of such crimes.

Over the last two decades these changed conditions have prompted world leaders, especially in the post-Christian West, to create a new formulation of just-war doctrine. What they have come up with is not a repeal of the medieval ideal, but rather an extension of it, all in an effort to shape the responses of nations to new, barbarous calamities.

The responsibility to protect, as this new body of doctrine is called, takes its name from the 2001 report of the Canadian government’s International Commission on Intervention and State Sovereignty. At the heart of this document was the principle that national sovereignty, far from being merely a privilege, carries with it certain responsibilities. These include the prevention of mass atrocities on native soil, military intervention in the affairs of an offending nation as a last resort and the active reconstruction of peace and justice in victimized societies.

As further elaborated by the African Union and, finally, by the United Nations in 2005 and 2009, the responsibility of every nation to protect its populations from genocide, war crimes, ethnic cleansing and crimes against humanity is deemed to have failed when a state can no longer offer security to its own people, or when it turns on them.

The military attack on Libya currently under way is, in my mind, fully justifiable by the principles I have outlined. The populations of an entire city, Benghazi, and of several smaller towns, are under imminent threat of mass slaughter — crimes against humanity — by troops loyal to Libyan leader Moammar Gadhafi. These populations must be protected.

I do not mean to minimize the grave risks involved in doing so. Innocent civilians may be killed or injured by the air strikes, causing fresh outbreaks of Islamic rage against Western “crusaders.” The West may also be exposed to charges that, while they are willing to aid Libyan rebels against a tyranny that has long been held in contempt by neighbours and the West, they have remained shamefully silent in the face of, say, Chinese repression in Tibet.

But like the Christian just-war doctrine in which it is grounded, the responsibility to protect is based on a highly civilized and profoundly Christian idea of justice. For that reason, Catholics can and should embrace this new set of principles and should seek to forward them in the community of nations.