But how to cope with such a danger clearly visible? Pin the electronic equivalent of a yellow star on them? Probably refugees would not object, but from historic POV hard to argue for over here. Create a vetting network here? Should be possible to at least start quickly and with swarm idea might actually be helpful using pictures of those going through official entry points, but within a few weeks terrorists would avoid official entry. Spying on own population also has bad aftertaste from WWII and east german times

You have a point. However, I am curious - how many refugees have the Muslim countries of Saudi Arabia, Turkey, etc accepted?