Just war theory is a venerable Christian tradition. It is the philosophical basis of international and American laws of war and undeniably noble in its intent. But it’s also deeply flawed, and the horrific Israel-Hamas war—to which many Western Christians have responded within a just war framework—demonstrates its limitations anew.

The basic elements of just war theory are two considerations: jus ad bellum (right to war) and jus in bello (right in war). As those phrases suggest in Latin and English alike, it’s about determining whether you have just cause to enter a war and whether you fight justly once the war is underway.

To answer those big questions, just war theorists ask many smaller ones. For jus ad bellum: Is war the option of last resort? Is it publicly declared? Is it declared by a legitimate authority? Is there a just cause? Is there a just goal? Is there a realistic chance of achieving that goal?

Then, for jus in bello: Is the use of force proportionate? Is sufficient care taken to avoid civilian casualties? Are prisoners of war treated humanely? Are war crimes punished by their own country? Is strategy set with an eye to de-escalation wherever possible and, ultimately, a just peace?

Just war theory isn’t monolithic, as probably no theory of this age and import could be, but the basics have been well established for centuries. A classic formulation comes from medieval theologian Thomas Aquinas in his Summa Theologica, building on the work of the early Christian thinker Augustine. You’ll find most iterations run much along these lines.

Just war theory is the intellectual ancestor of the Geneva Conventions—treaties dealing mostly with jus in bello questions that are central to the international law of war. The theory’s influence is also visible in how the US Constitution makes Congress, not the president, the legitimate authority to declare war. The rationale, as James Madison’s note taking at the Constitutional Convention put it, was “clogging rather than facilitating war [and instead] facilitating peace.” In other words, bring more scrutiny to the start of a war and it is more likely to be just.

Many later US laws of war, most notably the 1973 War Powers Act, are similarly informed by just war theory’s demands. Prominent modern Christian thinkers like C. S. Lewis and Reinhold Neibuhr also worked significantly within this tradition.

With such a lineage and so many queries in pursuit of justice, it may be hard to see why I believe just war theory is deeply flawed. Because, in one sense, there’s much to appreciate in this theory.

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Indeed, compared to most alternatives—and history is bristling with examples, but last month’s obscene Hamas attacks should provide sufficient contrast—the dominance of just war theory in the modern order is a remarkable achievement of Christian thought.

Given a binary choice between this and “a world in which there are no limits on warfare even in theory, and in which what can be done may be done,” as pacifist Catholic writer Tom Cornell put it at Plough—well, give me just war theory every time. And insofar as governments have promised to abide by just war principles, as the US government has, they should be held to that standard.

The problem is that the standard is manipulable. My core critique of just war theory is not primarily about hypocrisy, though there is plenty of that. It’s not merely that adherents say one thing and do another—that the theory’s stringent standards are often ignored by those pledged to uphold them.

It’s that the standards aren’t all that stringent. Just war theory can all too easily function less as a limit than as a malleable justification for whatever we’ve already decided to do. It needn’t be flouted because it’s more flexible than it seems. Like the legal expert to whom Jesus told the parable of the Good Samaritan (Luke 10:25–37), we often ask questions not to better love our neighbors but to justify ourselves.

“Ever since Just War theory was invented, every side of every Western war has used its language to justify self-interested claims, and done so with ease,” as Cornell observed. “After all, no government has ever announced its intention to wage an unjust war. … No victorious nation has ever attributed its success to its own evil deeds, nor have its leaders ever been indicted by an international tribunal for war crimes. That happens only to losers.”

Just war theorists in private life are not much better, Cornell continued. “Church leaders have had no better track record than the statesmen and generals,” he charged. “Throughout the ages, they have written a blank check to their governments on every side of virtually every war.”

There are exceptions, particularly in the jus ad bellum phase before a war begins. But I can’t think of a single US war in living memory in which a critical mass of American just war adherents deemed a war unjust in real time, before the verdict of history came in. And that category of adherents includes—at least in practice, even if they don’t know the theory’s name—almost every US evangelical who isn’t part of a historic peace church.

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Is that because our government always gets it right? Or is it because the theory’s standards are too stretchy?

The historical record suggests to me that phrases like just cause and legitimate authority and sufficient care to avoid civilian casualties are not mathematical formulas but judgment calls. And we are prone to judge in our side’s favor, to decide that our choices and those of our friends are justified, whether or not a dispassionate (or, say, omniscient) observer would agree.

“Christians cannot support violence if they feel that such support renders them liable to theological censure, if they feel that they are not doing the right thing,” in the words of the French theologian Jacques Ellul. “Thus acceptance of violence necessarily involves theological views; but these are formulated ‘after the fact,’ after the decision for violence has been taken.”

In direct contravention of its purpose, just war theory becomes retroactive justification rather than proactive restraint.

This is how former secretary of defense Donald Rumsfeld, even with a decade of hindsight, used just war theory to defend the 2003 invasion of Iraq—a preventive attack and regime-change project that notoriously included the use of torture, left hundreds of thousands of innocents dead, and almost eliminated ancient Iraqi Christian communities.

It’s how then-president Barack Obama invoked just war theory to explain his approach to foreign policy, while his administration used legislation from 2001 and 2002 to justify military intervention against groups in Yemen and Syria that did not exist when that legislation was written.

The more desperate a situation, the more tempting this kind of ethical elasticity will be. And the situation in Israel and Gaza is extremely desperate.

Unlike Hamas, of course, Israel did not start the current violence with a surprise attack on innocents. Israel is a partial signatory of the Geneva Conventions, has its own laws of war, and doesn’t fight in total disregard for civilian life.

But the Israeli ground assault on Hamas in Gaza, now ramping up, will be “fiendishly difficult,” as former US general David Petraeus told Financial Times. The most comparable modern fight may be the 2016–2017 Battle of Mosul, a fight against ISIS that took nine months—three times longer than anticipated—killed thousands of civilians and Iraqi troops, and displaced a million people. A compelling analysis at The Economist argues the war in Gaza will be even bloodier.

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The unavoidable reality, as counterinsurgency expert David Kilcullen explains at Foreign Affairs, is that Israeli ground forces will face “horrendously difficult tactical conditions, including room-to-room combat and tunnel warfare that would lead to massive casualties.” Kilcullen continues:

In Gaza, a key initial IDF [Israeli Defense Force] objective was to separate Hamas fighters from civilians. This was partly to protect the population and partly to identify legitimate targets. But this is one of the hardest aspects of urban combat, given that enemy forces are often dug in and embedded in noncombatant populations that, whether or not they support the adversary, become human shields. [In mid-October], Rear Admiral Daniel Hagari, the IDF spokesperson, stated that Israel’s “focus has shifted from precision to damage and destruction” in an effort to make Gaza untenable as a Hamas base. This suggests the IDF is placing less emphasis on avoiding civilian targets than before.

“We will do our best not to harm innocents,” Israeli ambassador to the European Union Haim Regev said shortly after Hamas’s murderous onslaught. “We are a democratic country. We are bound to the international law.” But Israel “will use all the means to eliminate Hamas and to rescue our people,” he said in the same breath. “You cannot fight against terrorists with your hands tied behind [your back].”

Tied, that is, by stringent application of the principles of just war. Less than a month in, the stretching of jus in bello has already begun.

Bonnie Kristian is the editorial director of ideas and books at Christianity Today.