https://www.cato.org/blog/syrian-refugees-dont-pose-serious-security-threat
Even the ultra-Libertarian *Cato Institute* explains why Syrian refugees are not a security threat. TL;DR: Vetting refugees takes a long time and is very thorough, and chances of actual terrorists getting through the years-long process are extremely low. There are other, *much* easier ways ISIS could get operatives into the US than hiding them among people who have left everything behind to escape a war zone.