Americans are joining ISIS — are they a danger to the US?

About 100 Americans and well over a thousand Europeans have joined the Islamic State in Iraq and Syria (ISIS). In theory, these so-called foreign fighters pose a major threat to their Western homelands: their passports give them freedom to slip back home, bringing their battlefield experience and anti-Western extremism with them, potentially to plot attacks. According to the New York Times, senior American counterterrorism officials believe “it is becoming harder to track Americans who have traveled there” as “the conflict in Syria and Iraq drags on.”

It’s worth taking a deep breath. The risk of Americans coming home from Iraq or Syria and pulling off an attack in the US, while something that intelligence agencies will and should take seriously, is something that Americans probably do not have much to fear from. While there’s more of a risk in Europe, there are still plenty of reasons to think that Westerners joining ISIS is far less of a problem than you might think.

About 2,000 to 3,000 Westerners have traveled to Syria to fight for one group or another during the conflict there, according to Thomas Hegghammer, the director for Terrorism Research at the Norwegian Defence Research Establishment. “The actual number of the people on the ground is lower” right now, he says, because “quite a few of them have returned.”

“So far,” Hegghammer says, “it looks like the blowback effect is going to be quite low.” According to Hegghammer’s research, at most one in nine Western jihadis who fought in a foreign war have come home to plot attacks. But he says that this estimate comes from a very low-end estimate of the total number of foreign fighters, and that the ratio that returns to attempt an attack in the West is likely “between one in 20 and one in 100.” To date, “the number is much lower” for volunteers from Iraq and Syria.

There’ve been “about four to six plots, involving five to ten people, in the West. And that’s out of 2,000-3,000 volunteers, so the blowback rate is one in several hundred,” Hegghammer concludes. Vanishingly small.

So far, there simply aren’t very many ISIS volunteers coming back West to plan terrorist attacks. According to Hegghammer, that’s partly because ISIS simply isn’t prioritizing an attack on the United States or Europe. They’re too busy fighting to preserve the caliphate to spend a lot of time abroad.

The conflicts in Syria and Iraq aren’t over yet. It seems likely that, when those conflicts start winding down or entering some sort of stasis, Western volunteers would become more likely to return home and stir up trouble.

But even then, they’re not nearly as dangerous as you might think.

“We’re going to know who these guys are, and we’re going to watch them closely as they transit home,” Will McCants, director of the Project on US Relations with the Islamic World at the Brookings Institution, told me.

McCants admits that’s it’s hard to catch ISIS volunteers on their way to Syria or Iraq. However, it’s much, much easier to identify them on their way home. “Once they’ve gone in,” McCants says, “US intelligence is going to find them.” Partly that’s because the US and other Western countries are obsessive about monitoring their borders and are keenly aware of this threat, but it’s also because jihadis love to talk on social media. “I’ve been told by people in US intel that publicly posted statements in Twitter are an absolute gold mine,” he says.

The Atlantic Ocean is the US’ friend here. Airports have the most “robust systems” for detecting returning fighters, according to McCants. It’s very hard to get back to the United States from Iraq or Syria without flying, and ISIS veterans who check in an airport will likely get detected pretty quickly. Once that happens, their numbers are small enough that US intelligence and law enforcement will be able to keep a very close eye on them.

The risk is greater for European countries, both because there are more Europeans fighting for ISIS and because those fighters don’t necessarily need to fly to get back home, making detection harder.

“The Europeans have robust systems in place when people travel through airports,” McCants says, “but it breaks down when people travel over land.” You could imagine ISIS militants driving through Turkey, for instance, or hooking up with smugglers to take them over the Mediterranean.

“I think there is a significant concern for Western Europe,” Clint Watts, a senior fellow at the Foreign Policy Research Institute, says. “They’ve got a big problem on their hands.”

But even if ISIS decides they want to plan a big attack in Europe, they won’t have an easy time of it. “It’ll be hard to stay off the grid,” McCants explains. Any terrorists trying a major attack are “going to be getting in touch with family and friends, or they’re going to need to get in touch with other militants to get materials.” The more time they spend talking and coordinating, the easier it is for European intelligence agencies to intercept their communications.

“The 9/11 attacks took a couple years to put together and pull off,” Watts says. “To expect ISIS to be able to pull something like that is not reasonable. I don’t get too stressed about it.”