The terrorist attacks on Brussels' airport and metro system that killed at least 31 people on Tuesday are the latest in a wave of large-scale attacks against European countries over the past 15 months.
Paris was hit twice last year. Then Turkey, which wants to join the EU, suffered several deadly bombings, the most recent of which happened just over a week ago. And then came the Brussels attacks.
As was the case with many of the others, the Brussels bombings were targeted, well-coordinated and designed to inflict maximum damage by striking packed public areas.
An offshoot of Al Qaeda took responsibility for the first Paris attack in January 2015, which targeted the Charlie Hebdo newspaper and a kosher market. The Islamic State (ISIS) has claimed responsibility for the second Paris attack, some of the bombings in Turkey and Tuesday's attacks in Brussels.
There has been one major ISIS-linked attack in the United States, in San Bernardino, California, last December. But that was perpetrated by two ISIS supporters, not skilled militants trained by the group.
The U.S. has been thought to be the highest-profile target for ISIS and is the most frequent target of its threats.
Memorials made after the attacks:
So why hasn't ISIS targeted the U.S. more directly?
A combination of factors explains why the U.S. hasn't been hit by a Brussels-style assault, experts say.
The most obvious factor is the U.S.'s "huge national security apparatus," Robert McFadden, retired special agent with the Naval Criminal Investigative Service (NCIS), told Mashable by phone from Washington.
Patrick Skinner, director of special projects for the New-York based Soufan Group and a former CIA case officer specializing in counterterrorism issues, hailed the U.S.'s "very strong border security."
Skinner also said the FBI "does a great job of disrupting [terror] plots early."
"Europe didn't do that for a long time. So they got these clusters," he continued, referring to the Brussels suburb of Molenbeek, which has become the center of attention for counterterrorism officials in recent months. Several of the Paris attackers were born in or resided in the area, and last week police found and arrested the last living one holed up there.
The U.S. simply doesn't have any such areas that are openly known as havens for terrorists, McFadden said.
Skinner said: "We don't have that level of extremism in the U.S. For all our faults, we haven't reached that critical mass."
But both McFadden and Skinner pointed to one major factor that stands out among the others: the huge number of European foreign fighters within ISIS's ranks.
The highest number per capita of European foreign fighters who have joined ISIS in Syria and Iraq are from Belgium, including many from Molenbeek, according to the Soufan Group, which for years has tracked their movements. Thousands more come from other European countries.
The number of Americans trained by the extremist group is only in the dozens, and none of them appears to have returned home.
Meanwhile, many of those originally from Europe have returned to their home countries after training with ISIS.
And then there are ISIS supporters who never left home, but who have been radicalized by the ones who did and came back.
"If you get 15 bad guys who stew in their radicalism, that's enough" to create a cell capable of carrying out a deadly attack, Skinner said.
Skinner and McFadden also cite another factor that has kept the U.S. from falling victim to a large-scale terrorist attack in recent years: luck.
"The truth is, we've been lucky we haven't had attacks on this scale. The very essence of terror is it's a sucker punch," McFadden said. "It's almost like we don't like to talk about it at times because we don't want to jinx ourselves. But it could happen anytime."
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