It has been widely reported that the outgoing Obama administration told the incoming Trump team that their most urgent problem would be North Korea. They were not exaggerating.

North Korea is probably the worst political regime on the planet: The vast majority of its 25 million inhabitants are best described as inmates. While the communist monarchy has worked hard to keep itself thoroughly isolated, reports trickle out, often from defectors, of horrific atrocities perpetrated by the regime against its own people.

For the outside world, however, and especially for the new U.S. administration, North Korea’s significance is tied to its increasingly frantic pursuit of nuclear weapons and the ability to deliver them. Since 2006, Pyongyang has conducted five underground nuclear tests; the most recent produced an estimated yield of 30 kilotons (about twice as powerful as the bomb detonated over Hiroshima) in September 2016.

No one is certain how many nuclear devices North Korea has right now, but they are widely believed to have enough fissile material to produce from 20 to 40 bombs, and production is ongoing. While they clearly have the ability to make warheads and possess short and intermediate-range ballistic missiles, they have yet to produce a warhead that could be mounted on a missile, nor have they successfully flown a genuinely intercontinental ballistic missile (ICBM). They are, however, testing components for several such rockets, some of which, if successful, could target most of the United States.

Experts believe that they could well achieve their goal in the next several years. While this is alarming enough on its own, North Korea is also a proliferator of weapons technology, selling its missiles to Iran, Syria and Pakistan, among others. North Korean scientists are even reported to have helped Iran with its own efforts to develop nuclear weapons.

Secretary of State Rex Tillerson announced during a visit to South Korea earlier this month that the Obama administration’s policy of “strategic patience” with North Korea was over and suggested that a new tougher policy was in the offing, one that includes possible military action. Strategic impatience is easy to understand, given the regime’s repeated violation of agreements reached in the past and seeming intransigence in the face of a series of economic sanctions levied by the United Nations since the 2006 test.

What does Catholic teaching about war and peace have to say about this grave threat faced by the United States and its Asian allies, particularly South Korea and Japan?

The Catechism of the Catholic Church contains a summary of the traditional teaching on just war. It emphasizes (echoing Vatican II’s pastoral constitution, Gaudium et Spes) that while the avoidance of war is an obligation for all citizens and governments, nevertheless, nations have a right to self-defense (2308).

The just use of military force must meet certain conditions, among which are a) that the damage inflicted by an aggressor be “lasting, grave and certain”; b) non-military means to end it have been shown to be “impractical or ineffective”; c) there is a serious prospect for success; and d) the use of military force must not produce evils and disorder graver than the evil to be eliminated. It is for the legitimate political authorities of a nation to evaluate the satisfaction of these conditions through their “prudential judgment” (2309).

Further, how military force is used is as important as whether or not it may be. Here, the most important moral principle is the immunity of noncombatants from deliberate attack (2313). Traditional just-war principles also caution against attacks on military targets that unintentionally inflict disproportionate harm on noncombatants.

These principles raise important questions for policy makers in the North Korean case. President Trump has, for example, suggested that he will not allow North Korea to successfully develop actual nuclear-armed ICBMs, suggesting thereby that he would intervene militarily to stop them. Such an intervention could constitute either “pre-emptive” or “preventive” acts of war. The difference between the two is important: A pre-emptive attack is one taken with moral certainty that an attack by the enemy is imminent. A preventive attack, however, is usually taken to mean that a threat exists, but is not necessarily imminent.

Most just-war theorists hold that pre-emptive military action can be justified under certain conditions, but that preventive war cannot, and this has been the position of U.S bishops, who held in a 2002 statement that pre-emptive war could only be just in the face of “clear and adequate evidence of an imminent attack of a grave nature.”

There is no algorithm for determining when such conditions are met — once again, the final decision must be made by political leaders in light of the best intelligence they can gather and accompanied by their sense of solemn responsibility both for the protection of their own community and for only the just and prudent use of deadly force.

The use of such force in the case of North Korea is complicated by many factors, among which is the particularly brutal and unpredictable character of the current North Korean leadership.

Kim Jong-un has continued the Kim Dynasty’s policy of internal repression, but also showed a particularly cold-blooded streak, murdering members of his own family (most recently his half-brother) and hundreds of other North Korean officials who have displeased him, including a vice premier, publicly executed with an antiaircraft gun in 2016 for falling asleep in a meeting. Kim and the North Korean ruling class live in a bubble of luxury, while most North Koreans go hungry.  That millions of his people could be killed in a war he initiates seems an unlikely deterrent, given his already-demonstrated indifference to their welfare. His frequent threats to use his weapons against the United States and its allies must be viewed in this light.

But even having resolved on a course of coercive disarmament, the challenges would be steep: Kim’s nuclear program is scattered over many sites in the country, much of it hidden underground and even under water, making air strikes very difficult to carry out successfully. If he actually succeeds in building armed rockets, any attack that did not achieve 100% success would allow the possibility of a counterattack against (at least) South Korea or Japan (not to mention the more than 60,000 U.S. military personnel based in those countries).

Even a conventional counterattack in response to a U.S. strike could be devastating, since South Korea’s capital, Seoul, a city of 10 million people, is a mere 35 miles from the border and well within range of hundreds of long-range artillery pieces already aimed at it. These factors must make any official weigh very carefully the consequences of military action in light of both the proportionality and likely success criteria of just-war doctrine.

There remains, of course, covert cyberwarfare against North Korea’s technology, which has already been used by the U.S., with mixed results, and the increased deployment of anti-missile technology, although the failure rate of those systems is worryingly high.

These risks have led many to hope that the North Korean arms program can be halted through a combination of negotiation and sanctions, such as those imposed by the U.N. since 2006 and other sanctions independently adopted by the U.S. and other countries.

Economic sanctions themselves carry moral implications. The Compendium of the Social Doctrine of the Church holds that such sanctions must have a clearly defined purpose, must be periodically re-evaluated for their effectiveness and impact on civilians, must be open to continued negotiation and dialogue, and must never directly punish a country’s entire population (507).

The international sanctions now in place do seem to meet these conditions, although they have not been maximally effective, as the regime’s recent actions demonstrate. Many authorities believe that more and more effective sanctions can still be imposed, along with far-tougher enforcement efforts, especially against international banks that facilitate both North Korea’s arms programs and their efforts to cheat on existing sanctions.

Much of Kim’s international financial activity is carried out with U.S. dollars and enabled by banks in Europe and China, so the U.S. could be much harder on those institutions. What must be avoided are sanctions that increase the suffering of the people of North Korea, an estimated 70% of whom are already malnourished.

The challenge posed by North Korea’s nuclear program calls for the very highest level of statesmanship. This includes the skills of coalition-building and maintenance with both our allies and adversaries in the region.

The governments of South Korea and Japan are essential players, but so is that of China, which has traditionally both protected and cajoled the North Korean regime in times of high tension (with a view to its own strategic objectives).

The current North Korean crisis is complicated by increasingly tense relations with China over its ambitions in the South and East China Seas.

One conclusion that seems widely shared is that Kim views the acquisition of usable nuclear weapons as crucial to the survival of his regime. The New York Times recently reported that U.S. intelligence agencies believe that Kim is motivated in part by the fate of Muammar Qaddafi, the late dictator of Libya, who gave up parts of his own nuclear program only to be overthrown with U.S. help and killed in the street by rebels.

There will be no easy solutions and no guarantees, but any policy must be measured against both the stubborn facts of the situation and the moral principles that set the boundaries of prudence and statecraft.

V. Bradley Lewis, Ph.D., is an associate professor of philosophy at The Catholic University of America.