The arrest this week of an Islamic State fighter in California, and a Yazidi woman's experience in Germany, are evidence of ISIS' successful infiltration of the West.

In the U.S. case, Omar Ameen was detained by the FBI after being identified as an ISIS fighter who participated in attacks in Iraq. In the German case, a young Yazidi woman who was previously held captive and repeatedly raped by the Islamic State told the BBC of her harrowing experience running into a former captor at a German supermarket. Both "Ashwaq" and her terrorizer, Abu Humam, apparently received refugee status from the German government. German authorities have been unable to trace Humam, almost certainly due to his possession of forged or manipulated identity documents.

Yet, regardless of what this means for refugee policies (I believe the West should heavily preference applications from women and children, not working-age men), these two stories evidence one simple, troubling fact: ISIS is in the West.

There are two specific issues here.

First off, there's the return of Western-passport holding ISIS fighters from Iraq and Syria. And while that's a counterterrorism concern, it has been mitigated by intelligence work to identify and arrest these citizens on their return. Those who have not been arrested are subjected to relatively extensive surveillance.

More concerning is the success of non-passport holders like Omar Amee and Abu Humam in accessing the West. Because if they're able to get into western Europe or the United States, they are, in intelligence terms, "clean skins." That is to say, terrorists who are unknown to Western counterterrorism services. And the scale of their access into Europe in particular (the BBC notes a German source who testifies that Ashwaq's experience is far from unique), indicates ISIS' success in procuring credible fake identity documents and cover stories for their infiltration teams. Benefiting from existing support structures in Brussels and other European capitals, these infiltrators give ISIS the means to conduct major attacks.

To be clear, while many of these individuals are no longer interested in jihadism, others are in the West to carry out attacks — either attacks as individuals, or attacks in the vein of the coordinated Paris and Brussels ISIS atrocities in 2015 and 2016. These directed terrorists are more dangerous than their homegrown/"inspired" counterparts in their superior operational skills and training to avoid intelligence service detection.

But where is the threat greatest?

Well, in Europe, it's Belgium, France, Germany, and Russia that are most at risk. Those nations have seen thousands of refugee returners from Iraq and Syria but lack the intelligence resources to adequately monitor all those of concern. Britain shares similar challenges, but more from returned passport-holders than foreign nationals. The U.S. is in the best position in that the FBI benefits from a vast resource base with which to investigate and intercept individuals of threat. Incidentally, the U.S. is also instrumental in preventing new ISIS attacks globally thanks to a boutique intelligence platform that I won't mention.

Nevertheless, it only takes one terrorist with a vehicle to carry out a murderous attack. And these two stories remind us that the collapse of ISIS territory in Iraq and Syria will not lead to the collapse of the group's ideology or its ability to carry out attacks.