Recent events in Rome indicate that some influential figures in the Vatican want Pope Francis to use his next encyclical to jettison the "just war" theory as the way the church determines whether or not it is moral to go to war. In particular, they urge that the church replace this age-old model — which focuses on determining a fight's justifiability by the degree to which it complies with criteria like necessity, likelihood of success, proportionality and discrimination — with a "peace movement" approach that comes very close to ruling out war as a legitimate instrument in any circumstances, and thus to pacifism.

Whether such a drastic shift is appropriate is, of course, in part a theological question. But it is also one of great consequence for the wider world.

The pope may not, as Stalin observed, command divisions, but he can move minds, and thus the political needle. Witness the impact of John Paul II on the collapse of communism or that of Francis on climate change or Cuba policy. A papal anathema of just war could, therefore, undermine important efforts to strengthen defenses in NATO and East Asia against a revanchist Russia, a rising China and a fanatical Islamic State.

Where does this push against just war come from? Advocates of dispensing with it argue that modern war is too brutal and too likely to escalate to be made compatible with justice. And, they contend, peace is more likely to result from a narrowing of acceptable uses of force.

Both of these arguments are fundamentally flawed.

First, the nature of modern conflict does not make just war an anachronism. Rather, its exacting criteria can still be satisfied. It is true that modern weaponry can be tremendously destructive, but it can also be used — if always imperfectly — controllably. Witness how the U.S. employed precision-guided munitions to wage war against Serbia and Iraq, or how the Pentagon plans to exploit technology in its "Third Offset Strategy" to ensure accuracy and effective command and control. No one should think that modern war is easy or susceptible to ready manipulation. But neither should it be assumed that it will necessarily spin out of control.

More important, a serious narrowing of the legitimate uses of force — let alone an embrace of pacifism — by the kinds of countries most receptive to such a call by the pope would be an invitation to the unscrupulous, the ambitious, the reckless, the aggrieved and the put-upon to press their claims, and press them hard. It would thus expose the world to more — not less — instability and ultimately war.

Coercion and aggression usually happen because one side thinks it can take something or compel submission and get away with it, or at least not suffer too much. Countries, especially hungry or revisionist ones, test limits. Saddam Hussein reckoned he could seize Kuwait and weather a mild storm. Kim Il-sung thought he could invade South Korea and avoid U.S. intervention. And, more recently, the Kremlin judged it could attack Georgia and seize Crimea without incurring a sufficiently painful response.

So what would happen if countries like the U.S. and its allies heeded such a call?

Leave aside for a moment the obvious problem of whether self-defense against invasion would then be legitimate. What about a Chinese blockade of Taiwan designed to bring the island it regards as a renegade province to heel? Would it be appropriate for the U.S. to use force to lift a potentially suffocating embargo of a partner or ally? Or what about in response to a Russian deployment of "little green men" in the Baltics to create havoc and thus a pretext for Moscow's intervention? These are not far-fetched scenarios.

If countries like the U.S. could not justifiably use force in these very plausible cases, what beyond moral suasion would prevent those unlikely to give much deference to any such papal call from seizing on the opening and pressing? Hitler does not lurk behind every geopolitical corner, but it seems reasonable to surmise that Moscow, Beijing, Pyongyang, Tehran and Raqqa, to name a few, would view this as an opportunity to exploit. Ambitious or dissatisfied powers are likely to press an advantage until they meet a firm counterforce.

Indeed, ultimately, such a restriction would have the paradoxical effect of making war more rather than less likely and conflict more rather than less destructive. Resolving to wait until threats are bigger and clearer invites them to become so and means they will be harder and nastier to counteract when they do. It is worth remembering that this was essentially exactly what happened in the interwar period.

Just-war theory is no doubt imperfect and frustratingly subjective. It has been used to bless questionable and even unjustified wars. But international politics is complex and morally fraught. It demands to be dealt with if even an imperfect peace is to be maintained. Embracing pacifism would be tantamount to copping out and would leave things to be resolved by those willing to deal with — or ignore — moral ambiguity. Dignifying too many wars would be casuistic, but dignifying none would be irresponsible.

Elbridge Colby (ecolby@cnas.org) is the Robert M. Gates senior fellow at the Center for a New American Security (www.cnas.org). He wrote this article for the Philadelphia Inquirer.