Trump has been talking about extreme vetting to reduce the chance of terrorists entering the U.S. The only way to vet people is through the visa application process. Currently the U.S. does not require visas for visitors from 38 countries, including France, Germany, Belgium, and the UK where there are domestic Islamic terror cells active. If the U.S. starts requiring visas from any of those 38 countries in order to vet visitors then those countries will require that we get visas in return.
At the rate those and other countries are admitting refugees, soon we’ll need to apply the same “extreme” vetting process to all countries, not just the 7 currently on the list.