In all the talk about whether to admit Syrian refugees, nobody I have seen has made what seems to me the most obvious argument. The U.S. hosts about sixty million tourists a year from all over the world. Does anyone seriously believe that any terrorist organization competent enough to buy or produce passports would find it difficult to get a dozen of their people in? That's about what a terrorist attack like the recent one in Paris requires.
As best I can tell, there simply is no practical way of preventing terrorists willing to die from killing Americans while doing so. Which makes the present antics of a majority of the House security theater.