When certain things happen, some people thing rigorous screening (of other people) will save us. We should screen refugees, would-be immigrants, mental patients, people with a history of violent behavior— and if we do it right, we are all 100% safe from everything.
Except that kind of screening isn’t possible. We can screen people on their histories, and on their current associations and intentions. We can’t determine what they will do in the future because even they don’t know it yet.
People change. Think of the ardent young atheist who goes off to college and becomes a Christian. Think of the young psychologist who thinks he can change everyone through therapy who ages into someone who thinks ‘lock them up and throw away the key’ about most criminals.
Some bad changes even make sense. Imagine some person of faith— Christian, Hindu, Jewish or Muslim— who comes from a more innocent part of the world. He is thrown into an American big city where he learns he must support homosexual behavior and homosexual marriage to keep his job, and if he is in the medical profession must support abortions and dole out abortion-causing pills— or else. He sees blatant prostitution on every street corner and learns that the schools will teach his kindergarten child about homosexual relationships and his teenage child that not having sex before marriage is weird and a possible sign of mental problems. Isn’t all that enough to drive a guy insane, possibly in a messy, violent way?
The United States has a long history of imperfectly screening immigrants. Even before the US was a nation, the people that came to these shores were not carefully screened, in fact, some were send as a punishment for criminal acts. And the Indians and the Pilgrims didn’t screen one another particularly well, or they probably wouldn’t have tried to live in close proximity.
We survived. And we will continue to survive. Because it’s not really unscreened immigrants who might or might not have terrorist ties that are the big risk we face. We could save more American lives by getting 10% more people to use their seat belts than we could by banning all new immigrants. We could prevent more mass shootings by giving 10% more law-abiding citizens the opportunity to get concealed-carry firearm permits.
But what really does endanger us as a distinct and mostly-good nation is when we let fear or propaganda persuade us to give up American values and American personal freedom in favor of measures that not make us safer, merely more supervised and controlled.