Today's refugees are not yesterday's refugees. The vast majority of refugees in the past who came from conflict zones were principally civilians and innocents caught up in conflict. Think the Jews in WWII. Today's refugees are often combatants or participants in various gross violations of human rights in very murky conflicts. Often, the government has no clue what these people did in the various conflicts they are coming from. As an example, police in Europe discovered on people's mobile phones horrific trophy photos and videos that migrants had brought with them (beheadings, rape, torture etc.). Then throw in the threat of terrorism. Jews were not trying to bomb aircraft or deploy IEDs to kill civilians in their adopted homelands.