So the argument that ISIS will pretend to be refugees as a means of getting into the US, obviously that's a desirable thing for them to do just to incite further hostility against refugees, but is it a more practical method of entering the US than whatever is currently available to them?
What's preventing ISIS supporters from travelling to the US at present? If potential attackers aren't making it into the states (I don't know whether they are or not) is it because border security is working or is it because there's limited attempts being made?
I'm trying to get a sense for whether accepting refugees could credibly increase the possibility of a terrorist attack in the US and if not, why not so I can respond to dicks on Facebook.
In the case of foreign fighters, I strongly expect that nations routinely share data about those they know and suspect have left to fight with ISIS.
In addition to this, I know representatives of Syrian groups in Europe also keep a close eye on refugees and try to account for their affiliations (as well as the Kurdish groups).
There will also be other screening processes. Who did the person come in with? Can other people vouch for their story? How far back do they know them? What are they carrying on them? How is their overall condition (well fed, decent quality clothing). Does their fitness level and pose suggest some form of paramilitary training? Where in Syria did they come from? What kind of funds do they have available to them?
The path most refugees take into Europe is also not one I suspect most terrorists would be willing to put up with. It involves a lot of walking, a lot of bribes, a lot of hiding in cramped and squalid conditions...not a whole lot of fun. I also would not be surprised if the people smuggler's did their own vetting...if they were found to have brought a terrorist into a country, they could well expect to end up in a far worse place than a prison cell for people smuggling. Some groups wouldn't care, of course, but even people smugglers like to think of themselves as being baiscally decent people, and have a future career that doesn't involve extraordinary rendition and simulated drownings.