Are our neighbours about to return from overseas to bomb and behead us? Suddenly, this is the great fear of every Western government. To understand the nature of that threat, and its limitations, you'd be wise to pay close attention to the suddenly infamous Islamic State fighter named John Maguire.

The 23-year-old Ottawa man, who flew to Syria last year on a self-professed martyrdom mission to fight with Islamic State (also known as ISIS or ISIL), is one of very few positively identified Canadians among dozens thought to have joined the rebel army. He has been described by reporters, and in his own busy social-media feeds, as fighting with the group to force an uncompromising Sunni theocracy on the Mideast. From Syria, he has turned to Facebook to call for the mass death of non-Muslims and Westerners.

In many ways, Mr. Maguire is as typical of Islamic State's foreign fighters as you can get. A dozen years of field research by intelligence agencies and scholars in Europe and North America has identified the characteristics that are most likely to lead Westerners to join Islamic extremist causes, and he holds many of them: He is from a middle-class family; he was born in the West; his family background is not religious and he didn't grow up in a Muslim neighbourhood; his childhood was marked by parental marriage breakup; he is highly literate but performed poorly in school; he expressed feelings of alienation from the world around him; he is socially awkward; he converted to Islam during a troubled time in his life out of political, rather than religious, motives; his radical views appear to have no support among most of his peers; he is highly active on social media.

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Of course, those qualities also describe an awful lot of non-extremist, non-violent Canadians, which helps explain why people like Mr. Maguire are causing so much alarm. We don't know how many there are, we don't know how to find them and we don't know what will happen when they return.

This has led some to warn that hundreds of Canadians and thousands of other North Americans and Europeans could be getting indoctrinated into anti-Western hate, trained in deadly techniques and prepared to return home from the Mideast to wreak terrorist violence.

Last month, Michel Colombe, the head of Canada's spy service CSIS, wrote in The Globe and Mail that there are "hundreds and hundreds of mostly young people" from the West who have joined "any number of fanatical groups that commit unspeakable violence in a global war against pluralism, democracy, human rights and gender equality," and that upon return, they could "attempt violent acts here in Canada."

This week, expressing similar alarm, British Prime Minister David Cameron proposed new terrorism laws that would give police power to seize the passports of suspected extremists and prohibit the return of foreign fighters to Britain, even if they are citizens. Some U.S. politicians have gone even further: "I think of an American city in flames … They are coming here," Republican Senator Lindsey Graham said last month.

But how real is the threat of such fighters in our own neighbourhoods?

On one hand, the threat is grossly exaggerated: Islamic State has shown no ability to carry out terrorist attacks beyond its small patch of the Mideast, and its membership numbers are perpetually limited by its extremist views, which alienate even most radical Salafis. So the prospect of Islamic State itself attacking the West is low. What's more worrisome is the notion of individuals, radicalized and trained, returning to commit acts of violence.

Even here, the numbers are very small. Mr. Colombe of CSIS testified before a Senate committee this year that there are estimated to be 130 men who have left Canada in recent years to join terrorist groups in Somalia, Yemen, Libya, Tunisia and Syria. Half of them have returned and about 30 are thought to have gone on to Syria and Iraq; many of those would have joined Islamic State.

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Those numbers have likely risen, but probably not by much: There is no mass constituency in any part of Canada for the extreme views of such groups.

In a detailed analysis of Syrian Sunni foreign-born fighters published last summer by the Washington Institute for Near East Policy and Flashpoint Partners, researchers concluded that the vast majority had come from Libya, Saudi Arabia, Tunisia, Jordan, Egypt and Lebanon (81 per cent of all fighters). Other sizable groups were from Chechnya, Russia and other former Soviet regions. Fighters were also identified from Bulgaria, Denmark, Ireland, France, Britain and the United States, but together, they represented less than 3 per cent of fighters.

Another study by the Foundation for the Defence of Democracies found that Westerners make up almost 20 per cent of Syrian fighters (including Islamic State and all other groups). The largest number come from France, Britain and Germany. North American fighters were too few to count accurately.

Those numbers are not zero, however, and should not be dismissed out of hand because they are so small. "Even given these relatively small numbers," the Washington Institute/Flashpoint authors concluded, "it would seem that the concern of Western governments that errant extremists from their countries will receive paramilitary training in Syria appears to be indeed borne out by the evidence."

And among the few fighters who do return from overseas adventures in extremism, there are indeed some who become extremists at home.

There was Mohammed Merah, the 23-year-old Frenchman from a moderate Algerian family who murdered seven people in Toulouse, including three children at a Jewish school, in 2012. He had become a violent extremist after having travelled to fight with the Taliban in Afghanistan. And there was Michael Adebolajo, the Englishman from a Christian family who beheaded off-duty soldier Lee Rigby in South London in 2013. He fell into Islamic extremism after having travelled to Kenya to fight with the al-Shabaab militia.

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In a major Dutch study of all the terrorist plots attempted in Europe between 2001 and 2009, both successful and thwarted, it was found that 12 per cent of returned extremist fighters had gone into terrorism upon their return. In another study, Norwegian terrorism analyst Thomas Hegghammer has estimated that the proportion of returned terrorist fighters who return to plot attacks is between one in nine and one in 20.

In other words, there is a good chance that at least one Canadian will return to attempt an attack here. While returning terrorist fighters are nowhere near Canada's top terrorist threat in terms of numbers, they should certainly be watched very closely by intelligence agencies.

This is where you'll find one small silver lining in this otherwise dark development: By going abroad to fight, such Canadians become very easy for intelligence agencies to notice, track and monitor. "We're going to know who these guys are and we're going to watch them closely as they transit home," Brookings counterterrorism scholar Will McCants told an interviewer this week. The fact that these fighters aggressively use Twitter, Facebook and YouTube as propaganda tools makes them even easier to find.

Plus, their mortality rate is very high, and rising. So viewed from another angle, by going abroad to fight, our extremists – already very few in number – are self-culling dramatically and rendering themselves far more visible to authorities.

They are a genuine threat, but not the largest or most ominous one facing us. We should be afraid, but we should not be very afraid.