This explains, in part, why Donald Trump is now President. He advocated a seemingly commonsensical approach to the terrorism problem during his presidential campaign, which was to ban Muslim immigration, including refugees.
But, in reality, every lethal terrorist attack in the United States since 9/11 has been carried out
by an American citizen or a legal permanent resident.
That's what makes the news that the terrorist who carried out the Manchester attack, Salman Abedi
-- the son of Libyan migrants who was born and grew up in the United Kingdom -- unsurprising.
Again and again since 9/11 terror attacks in the West have been carried out by second-generation Muslims who are citizens of the very country they are attacking.
This is also a key reason that they are sometimes so hard to detect or to stop. These terrorists are not interlopers from other lands -- but rather our neighbors.
We saw this phenomenon in the most lethal terrorist attack in British history since 9/11, which were the July 7, 2005 suicide attacks on the London transportation system, in which 52 commuters were killed. Three of the four suicide attackers in the London attacks were British citizens.
But why would a British citizen or an American citizen attack his or her own country? Why anyone would choose to murder complete strangers is something of a mystery, but typically these terrorists feel a kind of split identity: Not quite Western, but at the same time not quite at home with the culture of where their parents came from.
In other words, they don't feel like they fit in.
In some cases, this produces a kind of identity crisis, which is relieved by a non-practicing Muslim first embracing a more observant form of Islam, then adopting a more militant form of Islam and a tiny minority then moving on to violent jihad.
Many who follow this trajectory buy into Osama bin Laden's narrative that Islam is under assault from the West, and they must do something about it.
We saw this in the case of Major Nidal Hasan, who was born in Arlington, Virginia, and 39 years later killed 13 at Fort Hood, Texas in 2009. Hasan was a non-observant Muslim who came to embrace a militant form of Islam and eventually ended up as a jihadist militant. He even had business cards printed up that identified himself as "SOA" (Soldier of Allah).
A similar process happened to Omar Mateen, who killed 49 people at a nightclub in Orlando, Florida, last June. He was born in Queens, New York, and didn't grow up especially religious.
Over time Mateen's dreams of becoming a police officer faded and, at the time of the attack, he was working as a security guard at golf resort.
This didn't sit well with Mateen's heroic conception of himself, and he delved into the world of militant Islam -- pledging himself as an ISIS foot soldier when he carried out his gruesome attack.
According to a friend, Salman Abedi had grown a beard and begun to dress "Islamically," in a robe. These are often signs of growing piety, but mere piety doesn't mean a turn to militant Islam. The mechanisms of how Abedi came to embrace violent jihad are still unclear.
Abedi appears to be one of the many thousands
of Western Muslims who have embraced militant Islam, often as a way of trying to resolve the tension between their split identities.