No, we can’t “prove” that some Syrian refugees will turn out to be terrorists. Neither can the liberals prove that no Syrian refugees will turn out to be terrorists. But the likelihood that SOME Muslims turn out to be terrorists is 100%, and ISIS has proclaimed that it is infiltrating the refugee stream.
ALSO, the experiences in Europe makes it an indisputable FACT that there are terrorists in the refugee pool. It is impossible to “vet” Muslim refugees adequately, utterly impossible. There will be terrorists, and no Governor should be forced by the courts to put his citizens at risk.