After all, a group of eight gunmen from France and Belgium, all of whom were trained by ISIS in Syria, launched a series of attacks in Paris that killed 130 people on November 13.
Last week, the office of the U.S. Director of National Intelligence released its "Worldwide Threat Assessment,"
in which it noted that at least 6,900 militants from Western countries have traveled to Syria since 2012.
It was such veterans of the Syrian jihad that launched the Paris attacks.
But the likelihood of a large-scale attack by ISIS in the States is quite small because the number of Americans trained by the terrorist army is in the dozens, and none of them appears to have returned to the States.
This compares quite favorably to the thousands of Europeans who have been trained by ISIS, many of who have returned to their home countries.
Indeed, for the small number of Americans who have traveled to Syria to fight with militant groups. it has largely been a one-way ticket. Of the 23 individuals who have been publicly identified as having reached Syria, nine have died while fighting for ISIS or other militant organizations.
The Americans who died often lacked combat savvy -- such as Nicole Mansfield, 33, from Flint, Michigan, who was killed in 2013 -- or they died in a suicide attack as Floridian Moner Mohammad Abusalha did in a 2014 operation directed by al Qaeda's Syrian affiliate.
In the United States, there has been only one case of such a fighter trained in Syria returning and allegedly plotting an attack.
Twenty-two-year-old Abdirahman Sheik Mohamud of Columbus, Ohio, left for Syria in mid-April 2014 and fought there before returning home around two months later.
The government alleges that a cleric in al Qaeda's Syrian affiliate told Mohamud that he should return to the United States to conduct an act of terrorism and that he discussed some kind of plan (with an informant) to kill American soldiers at a military base in Texas. Mohamud has pleaded not guilty.
The threat in the States is almost entirely in the form "homegrown" self-radicalizing "lone wolves" who may find ISIS inspirational but have not been trained by the group. Indeed, the perpetrators of none of the deadly attacks conducted in the United States since 9/11 has received training from terrorist organizations abroad.
Carlos Bledsoe, who killed a U.S. soldier in Little Rock, Arkansas, in 2009; Maj. Nidal Hasan, who killed 13 at Fort Hood, Texas, the same year; the 2013 Boston Marathon bombers and the married couple who killed 14 in San Bernardino in December -- and who were inspired by ISIS -- were all so-called lone wolves, none of whom had training or direction from a terrorist organization.
Such lone-wolf jihadists have killed a total of 45 Americans in the United States in the past decade and a half; each of these deaths is a tragedy, but this is nothing remotely on the scale of the national catastrophe that unfolded on the morning of 9/11. Nor are these attacks of the magnitude of what unfolded in Paris in November.
The United States is protected first by geography. You can drive from Damascus to Paris. You cannot, of course, drive from Damascus to New York.
Second, it is protected because only a relatively small number of Muslim-Americans have signed up for the ISIS ideology and an even smaller number have actually made it to Syria to join ISIS.
All of this makes the chance of an attack organized by ISIS in the United States to be unlikely.
What is more plausible is that an American recruit inspired by ISIS might reach out directly to members of ISIS in Syria over an encrypted social media platform seeking some kind of specific directions for an attack. This would create a blended plot that was both inspired and directed by ISIS, incorporating elements of the San Bernadino attack and the Paris massacres.
We already saw a harbinger of this in May when one of the two ISIS-inspired American militants who attacked a Prophet Mohammed cartoon contest in Garland, Texas, sent more than 100 encrypted messages
to a terrorist overseas, according to the FBI.
Still, there is a natural ceiling to what kind of mayhem these individual American militants can accomplish without actual training from a terrorist organization.
launched by the two militants in Garland was a spectacular flop. Police killed them both before they could harm anyone.