NATO is the most powerful military alliance in human history: it combines three of the world’s seven nuclear-armed powers and tops it off with the conventional power of the world’s lone superpower. It has potentially formidable military powers within it: France, Germany, Italy and the United Kingdom could all well rearm to much success should they so choose.

And the Islamic State just set off bombs right near their headquarters.

A wholly reasonable and often asked question will be: Why doesn’t NATO do something? Why, when so powerful and so unassailable, doesn’t the mighty military alliance annihilate the Islamic State?

This article is intended to help frame those conversations as friends, family and Facebook randoms begin asking just that.

Reason #1: Because NATO is afraid of another Afghan or Iraq War

This is the overarching reason behind the reluctance. Both campaigns embarked upon a campaign of total annihilation of Al Qaeda: neither campaign really accomplished that. In the case of Afghanistan, the best outcome will be a weak, Western-funded “democracy” based in Kabul that spends most of its energy fending off internal corruption, rural insurgencies of varying stripes and convincing sponsors to keep the aid spigot going. After a few decades, such a state might well exploit enough of Afghanistan’s resources to become self-sustaining; until then, NATO will have to remain not far behind the Afghan state.

Iraq, meanwhile, is already close to worst-case scenario. There, NATO can only hope a Kurdish statelet will bring order to the north while a Shia statelet will conquer the Sunni tribes and manage them on behalf of the alliance.

In both cases, security is far from assured and NATO must occasionally deploy force to prop up their favorite allies. This bill has not yet been fully paid.

To destroy the Islamic State as it exists means to go into Syria. That means to prop up not two but now three weak states; it also means having to find a suitable replacement for the hated Assad regime.

Hardly an enviable task.

Reason #2: Because Islamic State terror attacks are often the power equivalent of a single NATO sortie

This is where the calculations go cold. When the Islamic State attacks soft targets in the West, they inspire terror, fear and hate, but they do not weaken the fundamental power of their enemies. This is a dark way to think of mankind, but it is a key reason why NATO has yet to rouse itself to a ground war over suicide bombs.

Compare the two displays of power: When NATO drops a bomb on Islamic State positions, it kills a geographically concentrated number of people; some are civilians, some are fighters. Rarely do those bombs kill hundreds; mostly, they’re in the low tens.

When the Islamic State launches a suicide attack, it has roughly the same effect: a geographically concentrated number of people are killed, often in the low tens.

A far more devastating attack would involve high-level military, government or economic targets: the World Trade Center in New York City was a hub of the American economic system, making its 2001 loss far more devastating and helping provoke the retaliation against the Taliban. But compare that with the same attack on the WTC in 1993; the towers remained and the net damage to the United States was so low that President Bill Clinton did not feel compelled to embark upon a war on terror.

Until the Islamic State is able to destroy such targets rather than attacking geopolitically inconsequential targets, it will be hard to convince NATO to spill the blood necessary to destroy it.

Reason #3: Any of these single attacks takes a huge amount of effort from the Islamic State

Because the Islamic State lacks a powerful conventional military and must hop from one foot to the other to avoid airstrikes, launching attacks further afield is a difficult and time-consuming task. As we have discussed, a single Islamic State attack in the West is much like a NATO bomb on Raqqa, except that the Islamic State must spend months and even years preparing while NATO has hundreds of thousands of bombs in stock, ready to be deployed.

To bomb Raqqa is therefore much less effort for NATO; it can continue to do so for a long time. IS, on the other hand, is slowed by the amount of effort it takes to carry out such an attack.

Were IS capable of launching regular attacks on NATO, it would justify a large war. But it can’t; instead, it must spend a much larger proportion of its power to carry out just one. That slows down the rate of attacks.

We’ve seen how this very rule limits war between nation states. In 2011, Cambodia and Thailand bombarded one another over a small temple on their border. Because use of military power for both states is expensive, escalation became unlikely; both states understood that arbitration was a better solution than conflict.

Reason #4: Because someone else is already doing it, though very, very slowly

NATO still seeks a united Iraq and Syria ruled by central governments from Baghdad and Damascus; the final form of those governments is less important than that principal. The thing is, both Baghdad and Damascus seek the exact same thing.

So long as there are anti-IS states in Baghdad and Damascus, NATO can sit back and allow other forces to wear down the caliphate. It has none of the drama and hardly any of the vengeance, but it’s far cheaper and probably more effective. While the fact that Bashar al-Assad remains the ruler of Damascus irks NATO, so long as his forces are around to contain and combat IS, NATO is far less compelled to begin a ground war.

Kurdish forces in Iraq and Syria as well are doing the same job: limiting the territorial grabs of the caliphate, wearing down through attrition the limited power of the Islamic State.

Should IS capture either Baghdad or Damascus, calculations would shift rapidly: suddenly, IS would no longer be as limited. That being said, such a conquest would almost certainly involve a much fuller, much larger Iranian and/or Saudi response: more reason for NATO to stay out, for if the Iranians and Saudis are willing to die to stop IS, why should NATO bother itself beyond an air war?

But that isn’t the case: IS has lost territory since 2014, mostly in Iraq, though a major offensive to retake Mosul remains such out of reach of Baghdad’s rulers. Meanwhile, Russian bombs have consolidated the Assad regime: though incapable of taking the fight to Raqqa, it has certainly kept the caliphate’s Toyota legions from the road to Damascus.

And until some or all of these conditions change, NATO will hold back

NATO’s citizens must overcome their fear of another Iraq or Afghan War; only a truly devastating attack, or the collapse of a regime in Baghdad or Damascus, could propel them to think so. So long as IS’ strategy hinges on soft targets, NATO’s people will be rightly outraged, but they will not see IS terrorism as anything beyond isolated, criminal acts, rather than strategic threats worth mobilizing society to wage war upon.

This is not necessarily the case for Turkey, whose strategic needs differ from the rest of the alliance. But to spur Western Europeans to put the required tanks and boots on the ground to Raqqa means changing the strategic situation in a fundamental way. Brussels, murderous and cruel though it was, does not yet change such a calculation. NATO will bring more warplanes to the skies of Iraq and Syria, no doubt, but its great armies will remain in their barracks.

This story first appeared at Geopolitics Made Super, March 23, 2016.