How would you even prove or disprove if something a 3 letter agency did stopped a future attack? What if it stopped the next 9/11? What if it didn’t do anythin

How would you even prove or disprove if something a 3 letter agency did stopped a future attack? What if it stopped the next 9/11? What if it didn’t do anything at all? How are you so certain of anything? I’m skeptical of everything. Read, verify, read counter arguments, formulate an opinion.