Many European countries have no terrorism problem. Why?

This map is fascinating.

Courtesy of The Telegraph's travel section, it offers a color-coded display of the global terror threat. Green countries, like Mongolia, Mexico, and Madagascar, are quite unlikely to experience terrorism in the near future. Yellow have some underlying risk, orange more still, and red nations have a comparatively high chance of experiencing a new attack sooner than later.

Now, the crucial word here is "comparatively." This is a broad category that at once encompasses Iraq and Syria — where encountering a band of Islamic State militants is a completely plausible scenario — and the U.S. — where terrorism is statistically a far less common cause of death than mundane things like walking, biking, choking, "any force of nature," and your local police department.

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Still, there's no denying some countries are safer from terrorism than others. Particularly in Europe.

Much of the region is a consistent, troubling red, but there are significant exceptions. Switzerland and Ireland are a low-risk green, while Portugal and Sweden are yellow, meaning they have only an "underlying" terror risk. These nations don't have the international import of their English, French, and German neighbors, of course, but they are geographically, politically, and culturally close — and yet much safer.

To explore why these disparate threat levels exist, and whether the high-risk nations can replicate anything from their low-risk neighbors, I reached out to three national security experts with differing places on the political spectrum but shared expertise on terrorism in Europe.

Robin Simcox, the Margaret Thatcher Fellow at the Heritage Foundation, focused on the "somewhat overly obvious answer" of demographics. "When you look at say, Switzerland, Ireland, Portugal, and Norway," he said, "their combined Muslim population is probably something just over 600,000. The Muslim population in the U.K., France, and Germany combined is around 13 million." (Simcox was estimating, but his numbers are quite close to CIA stats.)

Now, demographics aren't destiny. "It's not a fait accompli," Simcox continued. "Of course, the numbers don't translate to there being 13 million terrorists in the U.K., France, and Germany, but there is a very large disparity in the number of people groups like ISIS can target for potential recruitment, as compared to somewhere like Portugal, where the Muslim population is nominal."

It may not be nominal for long, if Portugal gets its way. Facing a low birthrate and an aging population, welcoming a younger population of Syrian refugees could make good sense for the Portuguese economy, and the predominantly Catholic country has followed Pope Francis' pro-refugee lead. Yet despite Portuguese Prime Minister António Costa's 2016 announcement that his country would accept 10,000 Syrians — double the number allotted by the European Union's refugee relocation program — fewer than 1,000 Syrian refugees have actually settled in Portugal to date.

Certainly, nothing like "a majority of Syrian refugees are terrorists or plan to do terrorism," Simcox noted, but he offered a word of caution to leaders like Costa who want to welcome more refugees from the Middle East: "The numbers need to be manageable," he says, if cultural integration and appropriate vetting are to successfully lessen the chance of terror.

Jonathan Laurence, a nonresident senior fellow at the Brookings Institution and professor at Boston College, also addressed relative Muslim populations, but took a different tack. "The key here," he said, "is whether or not you have second-, third-generation migrant-origin communities large enough for there to exist some kind of critical mass, where the kind of subculture could develop that at the very margins you find teens or young men in their 20s willing" to fight for ISIS at home or abroad.

But those would-be fighters may not be native members of the community. "It's important to note that converts are disproportionately represented amongst those who do go to Syria and Iraq in the last five years," Laurence continued, "so one can't say that being of Muslim background is necessarily a predictor. We see converts from the majority culture, from a Christian background, who are attracted to that death cult because it turns out their conversion to Islam wasn't a spiritual decision at all. It was a kind of a way to express themselves politically."

On that political front, Laurence pointed to two factors beyond demographics. First, there's Europe's imperial past, "unreconciled histories or open wounds that could fan" ideological flames, as well as a more current sense of political frustration and marginalization. These feelings aren't guaranteed to produce violence — none of these factors, Laurence emphasized, have a tidy, "linear relationship" with terror — but it could be fruitful in the long-term to reckon with these political legacies to "reduce the conditions where you have those feelings of alienation."

The second political factor is foreign policy. "The U.K. and France are both active internationally in prominent military coalitions that have targeted ISIS and al Qaeda," Laurence said, "and that in itself draws return fire, literally a kind of military tit for tat."

The foreign policy dynamic was central for Trevor Thrall, a senior fellow for the Cato Institute's Defense and Foreign Policy Department and professor at George Mason University. "My number one observation is that the pattern of attacks by ISIS, or ISIS-inspired attacks, follows reasonably closely with how involved those European countries have been in the U.S.-led coalition's counter-terrorism efforts in the Middle East," Thrall said.

He pointed, for example, to a table listing the number of troops each coalition nation contributed to the war in Afghanistan at the height of President Obama's surge in 2009 and 2010, and sure enough, "the countries that have suffered the most recently are the countries that had the most troops."

The safer countries were far less active combatants in the military side of the war on terror. Sweden and Portugal contributed a few hundred soldiers, while the historically neutral Switzerland is not part of NATO or the EU and is not fighting in the Middle East at all. Ireland sent just seven troops, "doing what, I'm not sure," Thrall commented, but "what they weren't doing was generating a lot of outrage. That, I think, is a key point. Violence begets violence. If you make the decision that the threat from ISIS and al Qaeda is such that it requires your country to take aggressive military action beyond your borders, you have to expect and grapple with the fact that there will be some kind of response."

This is not, Thrall conceded, the analysis many want to hear, and it is certainly not the analysis many in Washington want to share. But if high-profile attacks continue, it may gain a hearing yet. "Europe is riskier than it used to be in some places to some degree," Thrall concluded, "but frankly, the dial is in their hand. If they stopped intervening, they could turn down the volume — and very likely turn down the violence, too."