SPIEGEL ONLINE: How do you explain the history of the Islamic State (IS) to people who are stunned by its seemingly sudden rise to power?

Lister: In 1999, the IS father figure Abu Musab al-Zarqawi established a training base for his group in Afghanistan. After the United States invaded Afghanistan in late 2001, the group fled through Iran and ended up in northern Iraq. By 2003, it had effectively become Iraq's main jihadist resistance movement. During the US occupation of Iraq, Zarqawi made a name for himself and his group. It implemented sharia law to such an extreme level that the various tribal forces rose up and drove them out in a movement called the "Awakening". The group suffered significant losses at the time. When the US began initiating its withdrawal, it marked the beginning of an opportunity for a revival of Zarqawi's group. From about mid-2009 onwards, it began establishing a sort of shadow influence. It launched an escalating level of attacks against security forces, a campaign of intimidation against local officials -- within the military, police and local governments -- and one of extreme violence. The extent of the campaign created significant leverage for the Islamic State. It also helps to explain why the IS was able to take Mosul so quickly.

SPIEGEL ONLINE: You've said that the Islamic State has succeeded in doing essentially everything al-Qaida had previously done, only better, with the exception of carrying out a foreign attack. How do the two groups compare?

Lister: Both seek to establish an Islamic state governed by sharia law, but they have very different strategies. Al-Qaida has adopted a much more patient and long-term approach to implementing social control and governance, focused on creating the socio-political conditions for such a reality. The IS is much less patient in terms of this objective. Both in the mid-2000s and now, IS has always immediately sought to implement sharia law and govern the population just as soon as it takes control of a territory. Syria offers the best comparison in terms of strategy. Al-Nusra, the Syrian al-Qaida affiliate, has extensive influence across the country at a social level, but they did not choose to implement sharia directly until quite recently because they felt the social conditions weren't ready and that they would be rejected if imposed too soon. They have instead considered the long view.

SPIEGEL ONLINE: Would you say al-Qaida is somehow less extreme than IS?

Lister: Absolutely. Al-Qaida has been behaving relatively moderately in Syria -- at least compared to the Islamic State. That is starting to change, with Al-Nusra now perceiving a more hostile environment in Syria. It has begun imposing sharia in villages and towns in the northwest Idlib province, a development that has not been well-received by locals. The move is driven by a feeling inside Al-Nusra that it must acquire firm control of the territory.

SPIEGEL ONLINE: Is the fear that its fighters might otherwise desert and join the IS?

Lister: Yes. The Islamic State's extraordinary gains in Iraq and now in Syria have posed a significant threat to Al-Nusra. The IS is in a state where it can identify as governing the population, whereas Al-Nusra has little to show for itself. Al-Nusra is now trying to prove to its supporters that it is capable of governing and, indeed, that it is doing so.

SPIEGEL ONLINE: Is the IS actually trying to build a state? It seems as if it is trying to replace the government by providing social services such as subsidies for food and support for the elderly. How does IS organize this and where does the money for these services come from?

Lister: Money is key here. It is well-known that the IS is almost entirely self-financed. Its money comes from the control and illicit sale of oil and gas, agricultural products like wheat, the control of water and electricity and from imposing taxes within areas it controls. It is literally earning millions of dollars each week, and a great deal of this money is pumped into social services. This is symbolic of the fact that IS is presenting itself exactly as its name implies: as an Islamic State. To do that, you have to provide the same services a government would. People are allowed to keep their jobs, but they become employees of the IS as we saw when it temporarily had control of the Mosul dam. By threat of force, and by paying salaries, they have managed to retain professionals in their jobs both in Syria and Iraq, from waiters in restaurants right up to workers at the hydroelectric dam.

SPIEGEL ONLINE: You have stated that IS leader Abu Baqr Baghdadi has greater religious legitimacy than Osama bin Laden or Zawahiri ever possessed.

Lister: Even though they presented themselves as experts on Islam, neither bin Laden nor Zawahiri had official religious training. Baghdadi reportedly has a PhD in Islamic Theology and was also a cleric in a mosque in his hometown of Samara. That has allowed him to acquire an image of legitimacy as a religious authority as the head of the Islamic State. If you look at the individuals directly beneath Baghdadi, most are military and intelligence operators, professionals from the Iraqi military. That in itself shows the need to have a religious figure at the very top. Baghdadi doesn't appear to be a very charismatic individual, so clearly he is in his position because of his religious legitimacy.

SPIEGEL ONLINE: Who is in charge of the group's military strategy?

Lister: It's difficult to say with 100 percent confidence, but my perception is that military operations are being run by Baghdadi's immediate deputies. They are largely all individuals who previously served as officers in the Iraqi military or the intelligence apparatus, so they are much better equipped to design and implement this quite professional, methodological campaign in Iraq and Syria. I would be very skeptical that Baghdadi himself could have been the architect of that.

SPIEGEL ONLINE: How many fighters does IS have at its disposal?

Lister: The IS has around 6,000 to 8,000 forces in Syria and 15,000 in Iraq. The number in Iraq has increased substantially because they have co-opted many local men who were armed whenever they entered a town. They could either repent and hand over their weapons or join the Islamic State.

SPIEGEL ONLINE: IS seems to have a pretty sophisticated propaganda strategy, including the use of social media. Who do you think is behind it?

Lister: It's hard to know, but the use of these accounts certainly seems to be a coordinated strategy. In recent days, all the Islamic State media accounts have been vanishing overnight in European time zones. Twitter is removing these profiles every night and, interestingly, they were popping back up again the next day, suggesting a coordinated effort. However, the IS appears not to have abandoned Twitter altogether as an official platform, with much of this content being transferred to other, smaller social media platforms.

SPIEGEL ONLINE: How successful has the IS been in recruiting jihadists from Europe?

Lister: This conflict has attracted an unprecedented number of foreign fighters, and many of them come from Europe. The IS now has about 2,000 to 3,000 recruits from Europe. That could help if it potentially wanted to carry out attacks in Europe, but I'm not convinced it wants to do so at this point.

SPIEGEL ONLINE: In retrospect, what could have been done to stop the rise of the IS? Would the situation today be different if the West had armed moderate Syrian rebels at the outbreak of the civil war three years ago?

Lister: This is the subject of a huge international debate now. I believe the IS already began growing in Iraq in mid-2009, so in Iraq it would be close to where it is today even if the Syrian revolution hadn't happened. One can certainly debate whether arming the moderate opposition in Syria in 2011 would have helped to prevent the IS from taking control over territory in Syria. But personally, I think it was inevitable that the IS would acquire some territory in Syria -- with the revolution and civil war, the environment was ripe for it. Still, earlier support to moderate opposition could have significantly lessened the IS' ability to operate in Syria.

SPIEGEL ONLINE: Why did President Obama authorize military strikes in Iraq, but not in Syria?

Lister: The US has long held the view that the situation in Syria is too complex in regional terms for it to get involved. The US and Iran have opposing interests in Syria, overall, whereas in Iraq, they largely coincide. That's one reason the US has felt freer to intervene on some level in Iraq. Also, the US has interests in the Kurdish region of northern Iraq and long relations with the Kurdish Autonomous Government's peshmerga fighters. I don't expect that we will see much of an expansion beyond the limited air strikes we are seeing now and an extremely minimal footprint on the ground in order to ensure security for refugees. Nevertheless, I don't think it is sufficient to push back the IS. Air strikes have slowed but not weakened them. They're pushing back and will probably start to threaten Aleppo in Syria soon.

SPIEGEL ONLINE: What kind of relations does IS have with other jihadist groups in the region?

Lister: They are very minimal. The IS has no allies in Syria. It has several groups that have pledged allegiance, but in terms of bigger jihadist groups, it has few alliances. It has many in Iraq, but mostly with tribal groups and Baathists, not militants. By presenting itself as superior to al-Qaida, which has traditionally received support from other jihadist groups, the IS has isolated itself from wider jihadist structures.

SPIEGEL ONLINE: What will the Islamic State look like 12 months from now? And what will it mean for Iraq and Syria?

Lister: If the US doesn't escalate its intervention in Iraq, and conditions continue as they are, I believe the IS will largely expand the territory under its control in Syria. However, I don't see its relationships with other Sunni activists in Iraq lasting very long. They are not natural relationships and they only have the shared goal of overthrowing the government in Baghdad. Within 12 months those relations will have begun to erode, providing an opportunity for either a new government or the West to re-establish some of the old relationships that have evolved during the Awakening. We are already seeing that on a small level in Syria and signs of it in Iraq, where it could be replicated.

SPIEGEL ONLINE: What can the West do to help stop the Islamic State?

Lister: Unfortunately, the IS has been allowed to grow and develop to such an extent that any strategy to counter it will have to take years and require very significant resources -- and not just military ones. It will need to include social, economic, religious, political and diplomatic measures. We also have to find a way to end the Syrian conflict, which offers an open invitation to a group like this. And in Iraq, the government in Baghdad will have to start reaching out to Sunni tribes and bring them back into the system.