Why?
Did you disbelieve any of the numbers given in this paper?
https://www.cato.org/publications/policy-analysis/terrorism-immigration-risk-analysis
Let’s note, for comparison, that your odds of being killed by a shark (for an American) is about 1 in 3.7m, and the odds of being killed by lightning are 1 in 80k.
http://www.surfer.com/features/what-are-the-odds/#SXhkdef5AhjKLpKT.97
There’s a thing in South Korea where people think that you might get killed by sleeping with a fan on. They’re worried that if you turn to face away from the fan, a vacuum will form in front of your face, and you will suffocate in your sleep.
Now just imagine moving to South Korea and seeing the local politicians making all of this policy about how beds have to be shaped and writing up regulations on how to ventilation and A/C systems can be installed. And the government puts millions of dollars into ads that make sure everyone is aware of the problem and is careful about how they sleep.
Given that you know that exactly 0 people have died from suffocating in their sleep due to a vortex formed by their plug-in fan, wouldn’t you think, “This is fucking nuts”?
“Yeah yeah”, you’re thinking, “But that’s because we’ve got the CIA and NSA and everyone protecting us. It’s only so low because we have protections in place. Otherwise, terrorists would be sneaking in nukes every other day.”
Okay well so, let’s look at homicide statistics.
There are about 16k people murdered in the US every year (your odds of being murdered in your life is 1 in 270, btw, though it’s not relevant to my current point). But only about 10.5k murderers are arrested. The police are only able to catch about 2 in 3 murderers.
There are about 16.6 police officers per 10k citizens in the US. The total number of NSA and CIA employees is estimated at about 60k. Since they’re tasked with monitoring the whole rest of the world, that puts them at about 0.08 operatives per 10k non-American people. Their job is to find out bad things before they have even happened. A police officer already knows what happened and how, and simply needs to backtrack. The CIA and NSA need to gain information without being discovered and have no easy access to the people and places that could help them. The police have large capabilities to gain access to the locations that have the answers they need and rights to question just about anyone they want to, without fear.
So do you think that we should expect the CIA and NSA to have a greater or lower “win” rate than 2 in 3? Personally, I think that their job is much harder than the police’s and they have far less people to do it with. But even if we accept that they’re just as good at patrolling the world and are able to sense future crimes as well as police can solve past crimes, the amount of terrorist-caused deaths that they would be preventing would only be a 50% increase over what we already have. That moves your odds of being killed by a foreign terrorist from the range of a shark attack to…oh I don’t know, maybe a cow or something?
9/11 was an impressive single act of terrorism. But drunk drivers kill way more people than that every year. If you want to save your own life, the life of your kids, or whatever else, why not ban people from countries like Ireland or Estonia?
Factually, you’ll save a lot more lives. And it would still be an exercise in stupidity.