American intelligence agencies say there is no evidence that the Islamic State – which has yet to attack targets outside the Middle East – is plotting to hit the US, yet 71 percent of those polled by CNN last month said they believed the militant group had active terror cells within our borders. That disconnect results from the threat inflation that’s so deeply embedded in our discourse about terrorism.

So we should take sensational media reports and the dire warnings of fear-mongering politicians with a hefty grain of salt. At the same time, an unusually large number of Westerners have traveled to Syria and Iraq to join the militants’ campaign, and more level-headed analysts say that in the future, these fighters could return to their native countries and use what they’ve learned to mount attacks at home.

In order to get a handle on how serious that threat may be – and to evaluate the US strategy to “degrade and ultimately destroy” the group — BillMoyers.com spoke with Thomas Hegghammer, director of terrorism research at the Norwegian Defence Research Establishment, by phone from Oslo. Below is a transcript of our discussion that’s been edited for length and clarity.

Joshua Holland: You’ve researched the issue of Westerners heading overseas to join the Islamic State. Can you give us a sense of how many people we’re talking about?

Thomas Hegghammer: We’re probably talking about around 3,000 people from North America, Europe and Australia. There have been Western foreign fighters in other conflicts as well, but in much smaller numbers. In fact, there are more foreign fighters in Syria and Iraq today than the combined number in all previous conflicts combined. So it’s a really extraordinary situation.

Holland: You recently wrote in The Washington Post that not all foreign fighters are radicalized Muslims. Do you have a sense of what is motivating these people to join a group as violent and extreme as the Islamic State?

Hegghammer: First of all, people go for different reasons. Some are purely motivated by ideology or religion and are interested in joining a religious fundamentalist project, a kind of new Sharia-based state.

Others are more politically motivated. They want to fight the Assad regime, or they want to help what they see as the oppressed Sunni populations in Syria and Iraq.

And I think many are also pushed into this by various social factors. They’re intrigued by the adventure. This is a chance to go out and be part of what some might think is an exciting project. You can fire guns and be part of something unusual.

But the motivations for joining have evolved over time. In the beginning of the Syrian civil war, I think we saw more altruistic motivations – people wanting to go and help the Syrian opposition. Nowadays, we see more religious motivations — wanting to help build this Islamic State. And I think that not everyone who is there now signed up for what they’re into now. They went there at an earlier stage, when the Islamic State wasn’t as prominent and as brutal as it is today. So we shouldn’t assume that every one of these 3,000 people are hardcore Islamic State sympathizers.

Holland: Peter Neumann, the director of the International Centre for the Study of Radicalisation at King’s College in London, told the Financial Times, “In a context like ISIS in Syria, you have all these battle-hardy Chechens and then you have some fat guy from Luton turning up.” How effective do you think these foreign fighters are going to be in the conflict over there?

Hegghammer: Well, I don’t think they’re hugely important, militarily speaking. But they have another effect. For example, they can help raise funds for the Islamic State. They extend the international network of the Islamic State and put it on the map as the leader of the international jihadi movement.

The Islamic State can also use some of these people for particularly controversial tactics, like suicide bombings or decapitations. Foreign fighters are overrepresented, it seems, among the perpetrators of the Islamic State’s worst acts. So they help kind of radicalize the conflict – make it more brutal.

They probably also make the conflict more intractable, because the people who come as foreign fighters are, on average, more ideological than the typical Syrian rebel. So they’re less likely to compromise and to agree to ceasefires and that kind of thing.

Holland: I would imagine that intelligence services would be all over these people if and when they return. So while one can imagine a few slipping through and perhaps staging lone wolf attacks, it seems that a major, coordinated operation would be difficult. Your view?

Hegghammer: The dilemma here of course is that not every single foreign fighter will become an international terrorist, but some will. So how do we stop the dangerous minority without taking counterproductive measures against the majority? We can’t treat all foreign fighters as international terrorists. It can have a huge political backlash among them and their supporters. And in any case, what would we do with them? Europe already has a prison radicalization program. Putting another 3,000 semi-radicalized people in there could make that problem a lot worse.

So the question on every serious analyst’s mind is who among them is dangerous? What proportion of them poses a threat and how do we deal with that? And so far, the proportion is very small. So far we have somewhere between five and 10 plots in the West that are linked to foreign fighters in some way. We’re talking about maybe five or 10 individuals in total, out of an outgoing population of 3,000. So the proportion is still low, but it’s early in the conflict and people haven’t started coming back in large numbers yet. So a lot of people, including me, are worried about the long-term ramifications of this. Because we know from historical experience that foreign fighters are overrepresented in terrorist plots in the West, and they often make for more effective operatives. Plots that include foreign fighters have been more successful and deadlier on average than plots without them.

Holland: I want to shift gears and discuss the US-led intervention in Iraq and Syria. The US enjoyed early success using air strikes against Islamic State positions in Iraq. And I think our current strategy is based on expanding on those early successes. But won’t the Islamic State adapt its tactics to dilute the power of the coalition’s air strikes?

Hegghammer: Yes, I think that the enemy that we’re going to war against now is not the same that we’ll be fighting in a year’s time. We know from historical experience that rebel groups like this adapt to our offensives, and we’re already seeing signs that the Islamic State is lowering its signature, as we call it. That means blending in with the population, moving people and vehicles more discreetly, and basically being harder to spot and harder to target. So they’re gradually turning into a little bit less of a conventional army and a little bit more of an urban terrorist group.

And of course, therein lies the problem. While a lot of these Humvees and tanks that the Islamic State have will be scrap metal in the desert, the problem will be to defeat the kind of urban terrorist group that the Islamic State will revert to in a year or two. And that’s why I’ve argued that, yes, degrading the Islamic State is certainly possible, but destroying them is going to be a very difficult task, and I think it’s probably not a good idea for politicians to declare that as the objective. If that is the objective, then it’s going to be a very long and very costly war.

Holland: You have argued that this campaign may increase anti-Western terrorism. Why do you think that?

Hegghammer: The Islamic State and its sympathizers haven’t systematically targeted the West so far. There have been a few plots here and there, but they seem to have come from free agents — people who are just vaguely affiliated with the group. But the group isn’t maxed out on the capability side in the way that Al Qaeda has been. Now that we’re attacking them, it’s quite likely that both sympathizers and the Islamic State itself might want to carry out attacks to avenge the offensive. So I’ve called the air raids in Syria “kicking the hornet’s nest,” because by doing that, we make them very angry and more likely to attack us. And if they do, there will be intense pressure to deepen our involvement in the conflict.

Holland: You’ve argued that the US has had an unspoken, yet in your view, effective policy of deterrence against Islamic terror groups, and that this campaign is getting away from that policy of deterrence. Can you explain that?

Hegghammer: For the past seven or eight years the US has had a counterterrorism strategy based on deterrence. The idea is to only really go after those groups that attack the homeland, and use less force against those groups that don’t attack the homeland, those who operate only in the Middle East, for example. And this strategy stems from the fact that you just cannot fight all the groups at the same time. It’s simply impossible. So to maximize domestic security at minimum cost, the US has sent a message to jihadi groups around the world that if you come here, if you attack the homeland, then we will come after you. If you don’t, the pressure will be lighter. That’s why the heaviest repression has been in Afghanistan and Pakistan, where Al Qaeda Central is based, and in Yemen, where Al Qaeda on the Arabian Peninsula is based. These are the groups that have systematically tried to attack the West.

So by attacking the Islamic State before it has started to systematically attack the West, we’re diverging from that strategic principle and I think that’s problematic in the long term.

Holland: What kind of alternative policies do you think we might have adopted that wouldn’t include these risks?

Hegghammer: I think one alternative would be something more like containment, which basically means you encircle the enemy and cut off its sources of funds and weapons and hope that internal tensions will make it sort of rot from within. That seems like an unattractive option with the Islamic State given all the terrible things it does in the areas it controls. But I think it could be the least bad option. Use military force, for example, to stop the Islamic State near Kurdistan or keep it from expanding, and then target its financial operations — its oil sales, etc. — more systematically, and then use other, more subtle means to make it more difficult for the Islamic State to govern its territories, in the hope that the population eventually turns against them.

But it may be too late for that now that we’ve started bombing its headquarters. I think the important thing now is to create what I call “offramps” — to define circumstances in which the mission can be ended. If we proceed with the Islamic State’s complete destruction as our only goal, then I think it could be a very long war. We need a range of more limited objectives that we can actually reach.