During Iraq's long summer of 2004, one of the many prisoners who arrived at the American-run facility at Camp Bucca in southern Iraq was a young jihadist who fought under the name Abu Ahmed. Though he'd feared prison, Abu Ahmed found, to his surprise, a kind of jihadist salon, as extremist fighters locked up together spent their days discussing religion and military strategy.

There was one man in particular who stood out from the rest, Abu Ahmed recalled in an interview with the Guardian: a "quiet" but charismatic man who seemed driven by a desire for status and had a special authority over not just the other prisoners but even the guards, who allowed him to visit other camps. "You could feel that he was someone important," Abu Ahmed said.

That quiet man from Camp Bucca today goes by the name Abu Bakr al-Baghdadi and is revered by his thousands of followers as Caliph Ibrahim, commander of the faithful. He is the leader of the Islamic State of Iraq and Syria, a group so violent it was rejected even by al-Qaeda, and so grand in its ambitions that it now rules much of Iraq and Syria as a de facto state from which it is launching increasingly spectacular terror attacks abroad.

In the wake of the Paris attacks, a growing number of people are asking, with renewed urgency, about the group that has claimed responsibility. Who is ISIS? How did they come to be? What do they mean for the world, how can the world deal with them, and why hasn't it? What happened in between Camp Bucca and Paris?

What follows are the most basic answers to these most basic questions, written so that anyone can understand them.

1) What is ISIS?

The most direct answer to this question is that the Islamic State of Iraq and Syria (also called ISIS, ISIL, or Daesh) is a terrorist group that follows an Islamic ultra-fundamentalist ideology and that controls a vast region across Iraq and Syria.

But that hardly describes it.

It is a de facto state that declares itself to be the rightful heir of Islam's founding leaders. It considers itself at war with all nations and with all people who do not meet its standards as "true" Muslims. It believes its mission is to bring on the apocalypse as foretold in scripture. To this end, it seeks to conquer territory where it will build a real state and govern as it sees fit.

But for all its grand ambitions and twisted beliefs, ISIS is also a calculating, strategic organization that has brilliantly exploited the Middle East's political problems and social ills to recruit an army, win battles, and conquer territory.

Even more than that, ISIS is in many ways the ultimate culmination of problems that have been mounting in the Middle East for years: brutal dictatorship, religious extremism, sectarian hatred, foreign interventions, proxy wars, and a sense of hopelessness and anger among many, many people.

2) Where did ISIS come from? How did it become so powerful?

The two most important events in creating ISIS were the US-led invasion of Iraq in 2003 and the outbreak of the Syrian civil war in 2011. But to understand how all this happened, it helps to tell the story from the beginning.

Perhaps the place to begin this story is a quarter-century before ISIS formed, with the Soviet Union's 1979 invasion of Afghanistan, where Moscow sought to prop up the pro-Soviet regime that was under attack from rebels. Muslim foreign volunteers, seeking to repel the godless invaders, arrived to join the rebels (who called themselves the "mujahideen," which is just the correct Arabic word for "jihadis") — often backed by Saudi Arabia and by the US. Many of these fighters were Arabs who practiced an ultra-conservative version of Islam, rooted in and encouraged by Saudi Arabia, known as Wahhabism.

After the Soviet Union withdrew from Afghanistan in 1988-'89, many of the Arabs returned home, now veterans of combat who were ideologically hardened and infused with the belief that their faith in God had enabled them to defeat a superpower (not only had the Soviets withdrawn from Afghanistan, but the entire Soviet Union collapsed almost immediately after). Some of them, dedicated to a religious struggle they saw as global, formed al-Qaeda to continue the fight. They loathed both the brutal dictators ruling their home countries and the foreign powers that propped up these dictators while plundering the Middle East's resources. Al-Qaeda would in time declare war on them all.

Around this time, in the 1990s, a Jordanian man who fought in the Afghan jihad under the name Abu Musab al-Zarqawi founded an obscure terrorist group known as the Organization of Monotheism and Jihad. This would, several incarnations later, become the group that we today know as ISIS.

In September 2001, al-Qaeda launched the most consequential terror attacks in history, killing nearly 3,000 people in the United States. A year and a half later, after also invading Afghanistan, the United States led a massive invasion of Iraq. This invasion set in motion a series of events that would end up empowering extremism beyond even the worst of what many war critics had expected.

As the invasion sent Iraq tumbling into chaos, religious extremists rushed in, first to fight the American invaders and then to also wage what became a bloody civil war between Iraq's Sunni minority and Shia majority (the jihadists are Sunni). The US disbanded Iraq's Sunni-dominated army, now widely seen as a colossal error. Soldiers and officers, unemployed and aggrieved, joined the insurgents in large numbers.

Zarqawi and his group, which had slipped into Iraq shortly after the US invasion, distinguished itself as more violent, and more willing to slaughter civilians, than all others. In 2004, it joined in a sort of merger with al-Qaeda, with Zarqawi formally pledging allegiance to Osama bin Laden, and changed its name to al-Qaeda in Iraq (AQI). Zarqawi and al-Qaeda's leaders clashed frequently — often because the former was too preoccupied with killing fellow Muslims and too vicious even for al-Qaeda. This division remains today, and would years later lead to open war between Zarqawi's group and al-Qaeda.

Eventually, many of Iraq's Sunnis turned against AQI, and with significant assistance from US forces largely defeated them. Zarqawi was killed in a 2006 US airstrike. By the end of 2008, it looked like Iraq might have a real chance at recovering from the US-launched war.

But then Iraq's own prime minister, Nouri al-Maliki, committed a series of terrible errors. He allowed corruption and authoritarianism to deepen, persecuting political opponents. He reorganized the government to privilege Shias and marginalize Sunnis. When Sunni communities rose up in protest, he put them down brutally.

Maliki also gutted the Iraqi army, removing many experienced senior officers and replacing them with cronies. This happened as the US, under a deal negotiated by George W. Bush and continued by Barack Obama, withdrew US troops from Iraq. These two decisions left Iraq military incapable for ISIS's later rise.

AQI had, by then, merged with other groups under a new name, the Islamic State of Iraq. Its leader, a little-known and deeply pious Iraqi man who used the name Abu Bakr al-Baghdadi, saw an opportunity to regroup.

Also during this time, according to documents later uncovered by Spiegel's Christoph Reuter, a small group of senior Iraqi officers from Saddam Hussein's now-smashed police state constructed a plan to use AQI as a vehicle to retake Iraq. Their documents detailed a strategy uncannily similar to what ended up happening: The group would use Syria as a launch platform for a massive invasion of Iraq, where it would institute a system of control and extortion modeled on Saddam's Iraq.

When Syria disintegrated into civil war in 2011 and 2012, Baghdadi saw an opportunity. Encouraged by al-Qaeda's senior leaders, he sent a lieutenant to Syria to form the group Jabhat al-Nusra, which today is still fighting in Syria as al-Qaeda's branch there. But Baghdadi worried that Jabhat al-Nusra was growing too independent, and in April 2013 he did something audacious: He declared that he now commanded all al-Qaeda forces in both Iraq and Syria. He changed his group's name from the Islamic State of Iraq to the Islamic State of Iraq and Syria: ISIS.

Both Jabhat al-Nusra and al-Qaeda headquarters, though, rejected Baghdadi's claims. As ISIS moved its forces from Iraq into Syria, it began competing with Jabhat al-Nusra and other rebel groups for territory, with heavy fighting breaking out between them. From January 3 to January 15, 2014, just 12 days, more than 1,000 were reportedly killed in the infighting. In February 2014, al-Qaeda formally split with ISIS.

This was how ISIS became, rather than just another of many rebel groups fighting in Syria, something totally distinct. It focused not on fighting the Syrian government but rather on fighting other Syrian rebels for territory. It established control over a huge part of eastern Syria and, that summer, used Syria as a staging area to launch a full invasion of Iraq.

ISIS seized much of northern Iraq, including the country's second-largest city, Mosul. It declared all lands under its control to be a caliphate, subject to the authority of Abu Bakr al-Baghdadi, who dubbed himself Caliph Ibrahim. That is how, in the summer of 2014, this group came to rule so much of Iraq and Syria under a de facto state. It has since been at war on every front.

3) It seems like Syria's civil war is a huge part of all this. Is there a video briefly explaining the war and how it became so bad?

You bet. Watch especially for how Syria grows into a sprawling mess of a proxy war and how this creates space for ISIS to emerge:

4) Why does ISIS call itself a caliphate? What does that mean?

A caliphate is an Islamic state — and then some. In theory, a caliphate is more than just a country that happens to be officially Muslim; the caliphate is supposed to represent the entire Muslim community worldwide. The actual history of the caliphates is more complicated than that, but this is how ISIS's adherents see it.

The title of caliph, the leader of the caliphate, is not a spiritual authority in the way the pope is in the Catholic Church, but rather the political-military ruler of the Muslim community as a whole. Today, the idea of the caliphate evokes for many Muslims the idea of a glorious and unified Islamic civilization. This is a largely a fantasy that bears little resemblance to reality, but that fantasy is precisely the appeal.

When ISIS calls itself the caliphate, it's saying a number of different things: We are the only true authorities of Islam, we are the only legitimate government that rules over Muslims, we are the restoration of the glory days of Islamic civilization, and we are the beginning of the prophesied End Times.

The word itself, caliphate, comes from the earliest days of Islam's founding in the seventh century. The idea of a unified community of all believers is an important concept in Islam, which was founded in essence as a religion and a state, as Mohammed was not just the prophet of God but also the political and military leader of the first Muslims. When Mohammed died without naming a successor, his companions chose a man named Abu Bakr to be Mohammed's "caliph," or deputy. He and subsequent successors would act as the caretaker of the Muslim community on behalf of Mohammed. Thus, this community became the caliphate. It is no coincidence that the man who now leads ISIS chose to call himself "Abu Bakr" al-Baghdadi.

The present-day Abu Bakr al-Baghdadi, in declaring himself the caliph and his mini state the caliphate, is communicating that he believes he is fighting on behalf of all Muslims worldwide. He is also implying the desire to continue ISIS's advance until it has conquered all Muslim-majority lands, an aspiration frequently expressed in jihadist maps of a unified Islamic empire.

For jihadists, the caliphates are the height of Islam's glory, the banner of a sort of Islamic nationalism. Framing your jihadist movement as the rebirth or continuation of the caliphate is a way of asserting the idea that all Muslims should be joined in one state, that they should be ruled by Islam (or more specifically, the jihadists' version of Islam), and that all other Islamic authorities and states are apostates.

The jihadists also promote the idea that because the caliphates existed a long time ago and were politically organized around Islam, they must have therefore been ultra-conservative theocracies. But that is quite simply false: At their height, the caliphates were centers of artistic expression and scientific development.

5) What does ISIS want?

The standard answer to this question is that ISIS wants to rule and if possible expand its state, where it wants to enforce fundamentalist rule. But when my colleague Jennifer Williams posed that question to William McCants, a Brookings scholar who studies jihadist ideology, this was his one-sentence answer: "They want to restore the early Islamic empire called the caliphate and eventually take over the whole world."

ISIS is not going to take over the world. But that very earnest ambition, equal parts extreme and insane, speaks to the fervency of its ideology.

ISIS's first obsession, though, is not the caliphate but the apocalypse. The group's leaders, by every indication, earnestly believe that their role is to help usher in the final days and the end of the world. McCants explained, in his interview with Williams, how this belief developed into ISIS's focus on building a state:

In the organization's early history, they were not really focused on state building; they were much more focused on the end of the world, and they believed a savior figure called the Mahdi was going to appear at any moment and the great cataclysmic battle with the infidels was going to transpire. That made the Islamic State make some very poor decisions on the battlefield as a consequence, and over time the organization changed the nature of its apocalypticism. It focused much more on institution building as a fulfillment of prophecy — i.e., the caliphate — as opposed to the appearance of a messiah-type figure. That did two things for them: One is that it put their political program on a much more stable, long-term footing. But, two, they were able to maintain the apocalyptic expectations of their followers and potential recruits. They could argue that it was just around the corner. The first major stage in the end-of-days drama had been fulfilled with the appearance of the caliphate, and there was more to come — just not immediately.

This gives the group a clear mission: seize territory, subdue local populations, and expand. It also helps to recruit (more below on why people join ISIS). And by declaring itself a state and ruling totally over the populations in its territory, ISIS has a steady stream of income: extortion and "tax collection" from the people in its borders. It also profits from trafficking in things such as stolen antiquities and, often, in people: The group makes money from kidnapping schemes and from selling women into sexual slavery.

6) What's the relationship between Islam and ISIS?

This question often gets posed as some variation of "how Islamic is the Islamic State," but that question can mean many different things.

Sometimes people ask it to mean, "How sincere is ISIS's piety?" Among top leaders, very sincere. Abu Bakr al-Baghdadi is a real, trained Islamic scholar. Among recruits, it varies: Some are drawn by religious appeals, others aren't, and actual theological knowledge can range from high to zilch.

Or often people ask the question to mean, "How accurate is ISIS's interpretation of Islam?" It's a narrow and fundamentalist interpretation widely rejected by Muslims and Islamic institutions, but it is nonetheless grounded in real scripture. They're not making it up.

Often, though, even if they don't say it out loud, what people want to know is whether ISIS represents "true" Islam — and whether Islam is somehow to blame. CNN, infamously, aired segments asking, "Does Islam promote violence?" The answer to all of these questions is an emphatic no.

The vast majority of the world's Muslims despise ISIS; fellow Muslims are indeed by far their most common victims and their most committed enemies. The world's major Islamic religious authorities regularly condemn ISIS, as do Muslim-majority states — after all, ISIS has declared war on these institutions and states.

Religions are big and diverse, and people get out of them what they bring into them. For ISIS, this means an obsession with bringing about the apocalypse, with reviving medieval social norms, and with brutally punishing any perceived religious violation — all ideas that exist in the other Abrahamic faiths as well. But the vast majority of Muslims reject this interpretation of Islam and, rather, draw an interpretation that privileges peace and co-existence. So it simply does not make sense to blame Islam for ISIS.

That said, it is also wrong to pretend, as some in the US government do, that ISIS has "nothing" to do with Islam. The evidence that ISIS's worldview is derived at least partially in accurate and earnestly believed scripture is significant. And accusing ISIS members of being "false Muslims" just plays into their game: ISIS loves debates over who has the "truest" Islam and who can better quote scripture. It's more accurate (and more effective) to argue that ISIS's interpretation of Islam goes against fundamental human morality, and that mainstream interpretations of Islam are more moral and more suited to today's world.

7) Why do people join ISIS?

At first, in the years before it became ISIS, people joined the group and other jihadist groups in Iraq because they wanted to fight the American invaders, or because they were former members of Saddam's regime who wanted to retake power, or because they hated Iraq's Shia majority and wanted to terrorize them into submission.

Today, it's more complicated. There are a few overlapping reasons for why people, often traveling thousands of miles from their homes in Europe or elsewhere in the Middle East, are joining up with ISIS. There are the factors discussed above: People get excited about religious ideas of a coming "end times," or about joining in the revival of what they imagine will be a glorious caliphate, or they buy into ISIS's prolific internet propaganda portraying it as ever victorious.

Often, though not always, these motivations will be very personal and come down to some sort of identity crisis and search for meaning. Iyad el-Baghdadi, a now-exiled democracy activist from the United Arab Emirates, put this beautifully to my colleague Jenn Williams:

When you talk to them, there are many themes. There are themes of heroism, meaning, belonging, forgiveness, feeling like they have a purpose, feeling like they belong to something bigger than themselves. All of these are things that any young person goes through. It's just that they're finding the wrong answers. The people that are giving them the answers are basically the wrong people, the worst people. The fact is that rules give structure, and they give meaning. In the midst of all of this chaos around you, there are these rules, and they're defined rules and they make sense. That's why I call [ISIS's ideology] answer-focused. It's focused on the answers rather than focused on asking questions. That does have appeal. There are many young people, even young Muslims, who have chosen not to be religious at all, but they still appreciate answers. They want answers. And you can't beat a simplistic answer, rather just answering with another question. Because life is really a question, but these guys just want answers.

But, of course, there's more to it than that. People who study jihadism will tell you that certain factors increase the risk that a given person will "radicalize" and join a terrorist group like ISIS. Those include things like belonging to a marginalized community, especially if that community is overpoliced or treated as suspect, or if your society expresses hostility toward Islam or religion generally. People might also be more likely to join for more personal reasons, such as a sense of failure or a desire to belong. Many see the group as a path to adventure.

It is also difficult to ignore signs that some ISIS recruits join up out of a simple desire for violence. When ISIS posts videos of its beheadings and crucifixions, of burning a Jordanian fighter pilot alive, the goal might be to cement a certain narrative of an ultra-fundamentalist religious war rich with jihadist symbolism. But the effect is also to send an advertisement: If your greatest wish is to commit murder and to do it gruesomely, then join up with ISIS and you can do it.

ISIS also promotes what the New York Times's Rukmini Callimachi called a "theology of rape": a vast infrastructure of sexual violence and slavery, with ISIS encouraging rape as not just a tool of war but a matter of daily life in the caliphate. Yes, this is its own awful recruiting tool, but it's even more than that: It's a way for both the group and its individual members to demonstrate power by associating sexual violence with victory.

The group also recruits both men and women by promising pulp-novel adventure and romance. Erin Saltman, an expert in radicalization, told my colleague Amanda Taub that ISIS recruitment promises women a "strong Muslim man, who is a true Muslim, who is fighting for this very heroic cause," and promises men "young, nubile local women."

8) Whose fault is ISIS? How did the world let this happen?

The global failures that allowed ISIS to rise and flourish are many. Though none sought to create ISIS deliberately — the group has no allies and is an enemy to all — missteps and shortsighted policies by many nations have contributed to its rise. American allies, American enemies, and to some extent the US itself all share responsibility.

Bashar al-Assad: The one person who is most responsible for ISIS's creation is, ironically, the man from whom ISIS has taken so much territory: Syrian leader Bashar al-Assad. In 2003, after the US-led invasion of Iraq, Assad "took the view that jihad could be nurtured and manipulated to serve the Syrian government’s aims," terrorism scholar Peter Neumann wrote in the London Review of Books. His goal was to funnel jihadist groups into Iraq to bog down US forces there and prevent the US from threatening Syria as well. This helped fuel the rise of Iraq's jihadist insurgency, including al-Qaeda in Iraq (the group that would later become ISIS).

In 2011, after civil war broke out in Syria, Assad tried this same strategy again — but now in his own country. Assad released scores of jihadists from Syrian prisons and helped cultivate a jihadist movement within the rebellion. His strategy was to so tinge the opposition with extremism that the world would be forced to choose between him and al-Qaeda, and would thus not intervene to support the rebels as it had in Libya. It was ruthless and suicidal, and it worked: After Assad helped grow al-Qaeda's presence in Syria, the US and other foreign powers felt they could not safely back the rebels.

After ISIS broke off from al-Qaeda and began fighting against other rebels in Syria, Assad saw an opportunity: Here was a group fighting an open war against the rebels who were his greatest enemy. Unlike the rebels, whose first aim was to topple Assad, ISIS was focused rather on building its mini state in eastern Syria. So Assad chose to largely tolerate ISIS, letting it and the Syrian rebels fight one another. Though it's meant sacrificing much of Syria to ISIS's brutal rule, his decision has been successful, putting the Syrian rebels in a vice grip between Assad and ISIS's armies.

Iraqi leader Nouri al-Maliki: In his disastrous tenure as prime minister, from 2006 to 2014, the US-backed Maliki made the Iraqi government more corrupt and more authoritarian. After the US mostly left Iraq in 2011, he particularly tilted the government to favor Iraq's Shia majority and marginalize its Sunni minority. When Sunnis protested, he cracked down severely. This really exacerbated the sense of many Sunni Iraqis that the government was hostile to them or even illegitimate, and helped give jihadist groups, including the group that would later become ISIS, fertile territory to come back. The US finally helped push him out in 2014 in favor of a more inclusive leader.

Saudi Arabia and other Gulf Arab states: Contrary to popular misconception, Saudi Arabia does not support ISIS, nor do the other oil-rich Arab states of the Persian Gulf. "There is no credible evidence that the Saudi government is financially supporting ISIS," Lori Plotkin Boghardt of the Washington Institute on Near East Policy writes. These countries see ISIS as a major threat to them, and they're right: ISIS has already launched attacks in Saudi Arabia and Kuwait.

But Saudi Arabia and these other countries have long funded other jihadist groups in Syria, believing them to be the most effective fighters against Assad, whom they want to topple. This thus helped jihadism generally to rise in Syria, giving ISIS more fertile ground. Individuals in Saudi Arabia and other Gulf states have funded ISIS directly, wishing to support its extremist ideology. The Gulf governments have tried some to cut off this funding, but certainly not enough.

Turkey: The Turkish government, like Saudi Arabia, encouraged the rise of jihadist groups in Syria because it saw this as the best way to topple Assad. Turkey has a long border with Syria and has long allowed jihadists to move in freely. Once ISIS rose, Turkey didn't support the group but didn't fight it very hard, either — ISIS is at war with Syria's Kurds, and Turkey is fighting its own Kurdish rebellion and so was hesitant to fight the Kurds' enemy.

Iran: Bashar al-Assad's most important ally by far is Iran, which sees him as a crucial ally for projecting Iranian power in the Middle East and for countering its adversary Saudi Arabia, with which Iran is waging a regional struggle for influence. Iran has perhaps thousands of forces on the ground in Syria and appears at times to be running Assad's war for him. It is thus deeply complicit in Assad's actions that led to the rise of ISIS — even though Iran is simultaneously fighting ISIS in Iraq.

The George W. Bush administration: The 2003 US-led invasion of Iraq led directly to the rise of ISIS. It gave jihadists a compelling new cause — come fight the invading crusaders — and a huge pool of recruits by disbanding the Iraqi army. Without the Iraq war, Abu Musab al-Zarqawi's obscure terror group would never have grown into AQI, which later became ISIS. The invasion sparked the Iraqi civil war whose Sunni-Shia violence drove the sense of Sunni fear and panic that led many to support extremists. It created a sense among many Sunnis of a looming apocalypse that now forms much of ISIS's ideology. Because Saddam's Iraq had long been hostile to both Iran and Saudi Arabia, removing him upset the Middle East's tenuous balance and helped set off a cold war between Iran and Saudi Arabia that has led both countries to fuel Syria's civil war. And on and on.

The Obama administration: Experts and historians will likely debate for years whether Obama made a catastrophic error in following through on the Bush-negotiated agreement to withdraw US combat troops from Iraq: Would leaving a few thousand troops have been enough to stop ISIS from invading Iraq? Was it even politically possible for Obama to negotiate such a deal with Iraq, which badly wanted them gone? Even if the answer to both of these is "no," there were other failures, mostly of omission. The US waited until 2014 to push out Maliki, after the Iraqi leader had spent three years badly exacerbating the problems that helped lead to ISIS. The US also refused to intervene in Syria's civil war early on, when the opposition was less tinged with extremism and American involvement might have done the most good, only intervening much later, by which point there was less space for US bombing and rebel-arming efforts to help.

9) How can ISIS be stopped?

The world has actually had some real successes against ISIS over the past year or so. US-led bombing efforts, along with military campaigns on the ground in Iraq and by Kurdish groups in Syria, have taken away about 20 to 25 percent of ISIS's territory. Those are huge losses for the group.

Ironically, this is likely a big part of why ISIS has been lashing out with international terror attacks like those in Paris and the bombing of a Russian flight out of Egypt. It needs to maintain a narrative of victory to continue drawing recruits, and spectacular terror attacks are a way for it to appear victorious even as it's losing. It may also be about deterring foreign governments from attacking ISIS; the French have participated in airstrikes against the group.

But the truth is that the world's current strategy — bomb ISIS from the air while supporting Kurdish groups in Syria and the Iraqi army in Iraq — is just not enough. A well-worn but true cliché among foreign policy experts is that "ISIS is a political problem and a military problem." In other words, just fighting ISIS with bullets and bombs is not going to defeat the group unless the world can also solve the political problems that drive it.

Those political problems are in many ways the hardest part of the fight. There are, broadly speaking, three:

1) Syria needs a peace deal. As long as the Syrian civil war is raging, defeating ISIS there may well be impossible. Assad doesn't want to fight ISIS because he's more worried about rebels, and the rebels don't want to focus on ISIS because they see Assad as their real enemy. Meanwhile, the war is a big, chaotic security vacuum, exactly the sort of environment where groups like ISIS thrive. It also perpetuates a sense among Syrian Sunnis that they need protection from the Shia regime — even if that means turning to ISIS. So the only way to really take on ISIS in Syria is to first find a peace deal between the Syrian regime and Syrian rebels. After the Paris attacks, world leaders are pushing hard for this, but the remaining hurdles are huge.

2) Iraq's government needs to be fixed. Now that Maliki's been pushed out as prime minister, new PM Haider al-Abadi appears more inclined to reach out to Iraq's Sunni communities and build a more inclusive government. But many in Iraq oppose this, particularly hard-line Shia groups and the government of Iran, which wants a Shia-dominated Iraq and has deep influence in Baghdad. And even if Abadi does all the right things, Iraqi Sunnis have learned over the past decade to fear and distrust any Shia government — it's going to take a lot to bring them around. This one is much easier than the other two, but it's still very hard.

3) Iran and Saudi Arabia need to back off their proxy war. These two Middle East powers are engaged in something of a cold war for influence in the region, including by supporting competing sides in Syria. They are both big drivers of Syria's civil war — fighting one another to the last Syrian, if that's what it takes. That makes them both big obstacles to any Syrian peace deal and thus to defeating ISIS. The irony is that both oppose ISIS and are supporting the fight against it. But they just care more about fighting one another and about getting their preferred outcome in Syria. Until their priorities shift, or they're somehow pressured to withdraw from Syria, they'll continue driving the civil war that helps sustain ISIS.

Even fixing those things will not, on their own, defeat ISIS: The group still has to be fought on the ground. The US is currently supporting Kurdish groups in Syria and Iraq, and is supporting the Iraqi army in Iraq (which is also bolstered by Shia militias, many Iran-backed). But none of these groups are Arab Sunnis. ISIS is an Arab Sunni group that holds Arab Sunni territory. To defeat it there, Arab Sunni groups or militaries will need to retake and hold the territory. There's no one currently in a position to do that in either Syria or Iraq.

But then there are the deeper issues: authoritarianism throughout the Middle East, which pits dictators against popular movements and thus creates room for extremists to portray violence as the solution. Insecurity — from stifled economies, from scant political freedoms or social mobility, and to some degree from environmental factors — that robs people of hope for the future. Dictators and others who cynically exploit and exacerbate sectarian divides, pitting people against one another over religion or ethnicity. Disenfranchised and marginalized diasporas abroad, providing a source of extremist recruits. And, yes, foreign powers, including the US, all too willing to support friendly dictators or participate in proxy wars, knowing full well that any short-term gain this brings will perpetuate the same long-term problems they're looking to manage.

These are the problems beneath the surface that, even if ISIS can be defeated and eradicated, will leave the Middle East vulnerable to another horror like it. Such problems would likely take a generation to solve, perhaps more. There's not really any other way. But no one has any easy ideas for how to do it.