What is known in the history books as the Fertile Crescent and the Cradle of Civilization is now the world's most precarious battleground. Militants from the Islamic State of Iraq and Syria (ISIS), an al Qaeda-inspired Sunni Arab militia, are seizing resources and executing opponents in a dramatic push south deeper into Iraq and toward Baghdad.

Many fear that what is unfolding in Iraq could be the first salvos in a wider war that could engulf the Middle East and even redraw borders. Territory that lies between the Euphrates and Tigris Rivers is already falling to the fighters, and Shiite militias in the capital and south are gearing up for battle as ISIS closes in.

The advance of ISIS (also called the Islamic State of Iraq and the Levant, or ISIL) retraces long-held geographic and cultural divides in Iraq that separate the Sunni north and west from the Shiite south. Gareth Stansfield, a professor of Middle East politics at the U.K.'s University of Exeter, and a former senior political adviser for the United Nations Assistance Mission for Iraq (UNAMI), explores the history of the current turmoil and what it could mean in the months and years ahead. He was reached by telephone in Iraq this week.

Where are you right now in Iraq, and is ISIS anywhere near you?

I'm in Erbil. The ISIS forces are not far away, but there is a big Kurdish army in the way of them. Erbil is the capital of the Kurdistan region. I come here a lot, [as] this is one of the focal areas of my research. I had a prearranged trip that has just managed to coincide with these huge events.

But what is your security situation right now? Are you safe?

There's no threat from ISIS, as yet, toward this region. And the Kurds have a very big standing army that is pretty good, actually. It's not like the Iraqi army. So right now, with ISIS so focused on Baghdad, it would be pretty stupid of them to turn their attention up here as well, so I think it's pretty safe right now. I've been here a few days and should be here another week. In the '90s I lived here for five years. Been going back and forth ever since. I was a [U.K.-funded] political adviser to the Kurdish parties when they were building their own region and they were fighting Saddam.

View Images Members of the Kurdish security forces look at the wreckage of an Iraqi military vehicle on the outskirts of Kirkuk, on June 16, 2014. Photograph by Ako Rasheed, Reuters

As for Baghdad, why is it so key for ISIS?

It's both symbolic and strategic. It's the capital city and by far Iraq's biggest city. But what we have to understand about this insurgency is that it's not just ISIS. For ISIS, as Salafi-jihadists, attacking Baghdad is very symbolic because now it's now dominated by the Shiites, and they are engaged in sectarian conflict against the Shiites.

But the other side of this insurgency is not ISIS: It's the former Baathists, the former Saddam special republican guard and loyalists. For them, attacking Baghdad is very strategic. It's about recapturing the capital from what is seen as an Iranian-backed government. So they both have common cause together in this insurgency, symbolic and strategic.

The biggest of their targets—which are as important, if not more important to ISIS as a Salafist-jihadist organization—would be the Shiite holy cities of Najaf and Karbala. Right now the focus is Baghdad, but you don't even have to capture it from the perspective of ISIS. What you have to do is use Baghdad, and war in Baghdad, to really generate this sectarian conflict.

Arguably, Baghdad is too big to capture right now, even for ISIS. But you could start an absolutely catastrophic sectarian war, which could then bring in other forces that would inflame sectarian tensions elsewhere. It makes the whole situation even bigger than it actually is right now.

View Images Members of a Shiite militia train in the southern Iraqi city of Basra on June 17, 2014. Iraqi Shiite volunteers, who had been fighting in neighboring Syria, have been heading home to battle Sunni militants. Photograph by STR, AFP/Getty

Let's step back from the current conflict for a minute. Iraq's borders were drawn in around 1920—do those borders reflect the reality on the ground there, or [just] the imaginations of the Western powers that drew them?

It's a very big debate among academics, to tell you the truth. It was originally part of the Ottoman Empire and divided into three provinces, with Basrah and Baghdad in the south and center and Mosul in the north. There's a lot of debate among academics as to how much interaction went on between the three. Was there a notion of Iraq before Iraq? Some academics say there was, others that there wasn't. What I think is fair to say is that there were linkages between these three provinces, trade linkages. There was some population cohesion and geographic contact between different groupings, but there wasn't a state and there weren't boundaries, because at that time it was all part of the Ottoman Empire.

After World War I, the British and French began to connive—and the Russians too—to try to figure out where state boundaries would go. There was originally the Sykes-Picot agreement between the British and the French that came up with some boundaries, but not the boundaries that we see today. The San Remo agreement and subsequent League of Nations agreement saw the boundaries of Iraq and Syria we see today.

That brought together Baghdad, Basra, and Mosul for reasons really to do with Britain's interest. There was a need to bring Mosul into the equation because of oil, and there was the very important notion at the time that you needed to have Mosul and Baghdad together because they're Sunni dominated—in order to balance the Shiite predominance in Basrah. That sectarian agenda was there from the outset. The British wanted to include Mosul vilayet [the Ottoman Empire word for province], which includes the mountains of Kurdistan, largely [for] reasons of imperial necessity. Defending Iraq from the mountains required perhaps a third of the number of forces as it would have from the Mesopotamian plains. The British were pretty much bankrupt after World War I and didn't have the resources.

Why did the British originally choose to put the Sunni in power?

The British had had a long engagement with Sunni Arabs in the Gulf and worked with Sunni Arabs in World War I. Lawrence of Arabia was working with Sunni Arab tribes at that time, and there was just a mistrust among the British about Shiite Arabs: They were seen as more difficult to work with. Would the state of Iraq have emerged without the British? I very much doubt it.

Over the 20th century Iraq matured; the narrative of Iraq developed under the monarchy, then under the republic, then the Baath. But it was a very exclusive version of Iraqi nationalism, one that didn't have a place in it for the Shiite religious narrative or the Kurdish nationalist position.

Now of course the prime minister, Nouri al-Maliki, is Shiite, so that narrative had changed. But looking at the map we still see divisions—Shiites in the south along the Iranian border, and Sunnis more to the west and north. And then the Kurds up north.

The first time you hear of the Kurds referred to historically would have been about the time of the Crusades. Notions of Kurdishness, politically, really begin to appear in the 15th and 16th centuries, and by this point there are different emirates and principalities. They're principally Sunni Muslims, but they have a lot of different minority religions as well. Kurdistan had been this region in the mountainous area that became Turkey, Iraq, Iran, and Syria, and for several hundred years there's been Kurdish distinctiveness there. The area south of that—since time immemorial—has had mixed populations of all sorts of peoples on the Mesopotamian plains all the way down to the Gulf.

Iraq is one of the oldest centers of civilization in the world, all the way to the Sumerian civilizations seven to eight thousand years ago. But you then have the homogenization of Islam following the revelations of the prophet Muhammad. In the early days of Islam, this area was very much part of the Islamic caliphate. Then the schism happened.

View Images An Iraqi man tries on military gear at a shop in the city of Basra on June 16, 2014. Photograph by Essam Al-Sudani, Reuters

How did that schism turn one group into Shiite and one group into Sunni?

The struggle was over the successor to the prophet Muhammad, whether succession would go through the line of the family of the prophet, through the sons of Ali—he was Muhammad's son-in-law who was married to Fatima, his daughter—or whether it would rest with the political successors to Muhammad, the caliphs. It became about succession, and this came to a head in the battle of Karbala in A.D. 680, when the sons of Ali were trapped and defeated in battle and killed. "Shiat Ali" [the historic name for the Shiite] means "supporters of Ali." Since then the Sunnis have viewed the Shiites as heretics, and the Shiites view the Sunnis as the killers of Ali's sons.

We see ISIS taking hold in the north, which is more Sunni, but they are headed south. Why did the Shiites mostly end up in the south of the country?

I think it had to do with the Shiite holy cities in Karbala and Najaf. We have the impact of having those two big seminary cities based there that were promoting conversion in later centuries. As the Ottoman Empire was settling tribes, those in the south would be converting more freely to Shiism as well. Some of these tribes were also resenting the centralization by the Ottomans, who were Sunnis—resenting the way they were being treated, particularly in terms of land ownership and their economic position. So they were converting in protest to these Sunni Ottoman overlords as well.

So we then start to see this pattern of Shiism emerging, and of course in Iran they converted to Shiism quite some time before, and there is that influence too. So these tribes in the south, with Karbala and Najaf proselytizing, were not happy with their Sunni overlords and were adopting Shiism as a protest.

If you take the rest of Iraq, Baghdad and Mosul were very strong Ottoman Sunni Arab towns—Damascus as well. This was a triangle of strong Sunnism, compared to the influence of Karbala and Najaf in the south. The Ottomans in some ways got it right. This sectarianism was there, but wasn't as vitriolic as we see it today. It would be quite acceptable within the Ottoman system to have a whole range of religions existing side by side, but there was this increased politicization of Shiism from the 18th century onward caused by land reform and economic reform issues.

View Images Shiite fighters march during a military-style training in the holy city of Najaf, on June 16, 2014. Photograph by Ahmad Mousa, Reuters

When we look up north toward the Kurds, is there a concern ISIS could spread there if they defeat the south?

ISIS isn't in Kurdistan, but they're in the disputed territories. This is the area of Iraq the Kurds claim should be in the Kurdistan region but is not right now. With ISIS attacking so heavily and defeating the Iraqi military, the Kurds have moved into controlling most of Kirkuk and a lot of Kurdish-populated Nineveh, which means the KRG [Kurdistan Regional Government] now has a border with ISIS. It's about 1,000 kilometers [620 miles]. ISIS is not in the Kurdistan region, but they're right at the border of it.

The Kurdish military have been warning about the threat of ISIS for a long time because it was very clear that ISIS had become very strong in Mosul. But this is a sectarian war between Sunni and Shiite. The Kurds need to defend their boundary and watch what's happening, but it's not their fight. That doesn't mean ISIS will leave them alone. If ISIS wins in Iraq, for example, then I could imagine they would turn their attention to the Kurds very quickly.

View Images Iraqi volunteers leave the main military recruiting center in Baghdad to join the Iraqi army, on June 14, 2014. Photograph by Karim Kadim, AP

Which natural resources does ISIS now control?

They don't have many natural resources in Iraq. They've been exploiting some oil fields in Syria for some time now, illegally. But the northern Iraqi oil industry is in Kirkuk, and the Kurds control that. ISIS has a place called Beiji surrounded, which is between Mosul and Tikrit and is a huge refinery. If you control Beiji, then you basically control the petrol supply of Iraq, or that part of Iraq at least, which makes it extremely strategic. You'd control the pipeline that comes down to Beiji and then north to Turkey. So it's a strategic pipeline you control as well.

Their advance has been pretty spectacular. What's probably holding them back is that the farther south that they go, they will consolidate the opposition against them. They'll be coming across Iraqi security forces that may realize they have no choice but to stand and fight. These guys are very serious. If Baqubah falls, they then get to a string of Sunni towns that were put in place by Saddam. This is interesting political geography: Saddam built a whole series of Sunni-dominated towns around Baghdad, around the north and south, in order to isolate it from potential threats from other communities.

Until 2006-2007 Baghdad was at best a mixed city but probably Sunni dominated. What happened from 2006 onward was the sectarian uprising in Baghdad, to the extent that it is now predominantly Shiite. Most of the Sunni neighborhoods became Shiite. Right now we have the mobilization of the Shiite militias again that were not part of the Iraqi security forces, but were part of these informal militias that were fighting the civil war in 2007. It's with these groups that the Iranians will potentially work as well.

This gets us to the question, what does ISIS want? If ISIS simply wants to control Sunni-dominated territory, then they stop. But ISIS is a Salafist-jihadist organization that wants to attack the Shiites, so why would they stop? ISIS [fighters] don't care if they die. If they die fighting Shiites, then that's martyrdom. So why not attack them? It depends on whether the more secular heads of the Baathist side come into this and start to think more strategically about this game than the Salafist side of this.

View Images Members of the Kurdish security forces detain a man suspected of fighting for ISIS, on the outskirts of Kirkuk on June 16, 2014. Photograph by Ako Rasheed, Reuters

Do you expect any changes to borders or any longer-term geographical changes as a result of all this?

In terms of the geographical side of this, the rise of Kurdistan is an absolutely critical one. What has stopped the Kurds of Iraq moving toward independence in the past is that their own nationalist aspirations have never been aligned with the wider regional system or the international system. Now their national aspirations to be independent are aligned with the region, and the animosity toward the Kurds in Iraq and the rest of the Middle East and Turkey is gone. Turkey is the biggest partner to the Kurds in Iraq. Ankara and Erbil have just signed a bilateral oil export agreement for 50 years.

With the Kurds being reasonably peaceful, trying to be democratic, and now having to support all of these refugees, being at the front line with radical Islam, and being favorable to the West—their interests now very much clearly align with Western interests and with regional Middle East interests. So now there is no reason why the Kurds can't push their agenda for independence even more. This is a new state forming.

View Images After fleeing the violence in Mosul, a boy stands near tents in a refugee camp close to Erbil in Iraq's Kurdistan region, on June 14, 2014. Photograph by Jacob Russell, Reuters

I know it's hard to predict, but what do you think we're going to see a few months from now in Iraq?

It is very hard to predict. Personally I think you see a catastrophic civil war among Sunnis and Shiites. When you have an organization like ISIS dragging people out of cars, asking them to prove that they are Sunnis and not Shiites, and beheading them when they can't prove that, then you've got a clear problem. When 1,700 men get executed all at once for being Shiite, that's a very big sectarian problem. There are crucifixions going on in Syria by this group. When you have this level of atrocity—this is like former Yugoslavia on speed—you can surely bet that the Shiite militias will be just as harsh when they come back. They're not averse to being full-on sectarian as well. It's very difficult to rewind these sorts of atrocities.

The ancient battle of Karbala and the killing of Ali's sons becomes very real and very powerful in the modern setting, especially when both sides want to push it. It doesn't matter if Western observers sit on the sideline and say there are still forces of moderation in Iraq. I think it's way beyond that. The rhetoric on both sides is extremely inflammatory, extremely sectarian, and the atrocities that are happening every day are just furthering that agenda.