> Because terrorist attacks that were prevented didn't happen, there is no way to know. We could only speculate by correlating terrorist attacks and surveillance, but there are so many external variables that it is effectively impossible to prove causation.
This is nonsense, foiled attacks make the news. The FBI foiled a terrorist attack against the governor of Michigan a few days ago and that certainly made the news.
There is a difference between a foiled attack and one that is prevented.
For example if you lock your bike and when you come back, you see bolt cutter marks on your lock and your bike it still there, you know that it stopped the thief.
But for one attempt like that, there are probably a hundred more potential thieves that just passed by, saw your lock and didn't do anything.
Seeing your lock untouched doesn't mean it is useless. It may or may not have prevented your bike from being stolen, but there is no way to be sure.
In the same way, a terrorist may have had plans, but because of surveillance, he knew he would never have succeeded, and gave up. Seriously I doubt it, but that's a possibility.
This is nonsense, foiled attacks make the news. The FBI foiled a terrorist attack against the governor of Michigan a few days ago and that certainly made the news.