I realize the statement was ambiguous between &quot;prevent some attacks&quot; or &quot;prevent all attacks.&quot; It was an attempt to state one position taken by those actively involved in the debate: multiple terrorist attacks have still occurred (Paris, Boston marathon, etc), despite massive growth of the national security complex. Which makes discussion of the efficacy of intelligence agencies relevant to current events - my response to the comment I was replying to.<p>But to answer your question, to the best of my knowledge, it&#x27;s widely reported that there is no evidence of terrorist attacks that have been prevented. NSA claims of 54 terrorist attacks being prevented are widely reported as false[1], mass surveillance and security theater is considered by (at least some) experts an ineffective way to stop terrorists[2], and most of the people arrested for terrorism seem to be mentally ill individuals entrapped by law enforcement[3], as well as a whole host of other sources effectively implying &quot;none&quot; that I don&#x27;t think are worth taking the time to exhaustively cite. You can always search for &quot;terrorist attacks prevented by NSA.&quot; If there are any examples, I&#x27;d be interested in knowing.<p>Alternatively, it could be that efforts of law enforcement raise barriers to terrorism such that much is prevented indirectly, but that would be a speculative conclusion until there&#x27;s some evidence it&#x27;s actually the case. Successful terrorists able to plan and communicate in plaintext using SMS and discussing their goals in their English-language propaganda (Dabiq) doesn&#x27;t really lend credence to the preventative costs argument, either.<p>[1] <a href="https:&#x2F;&#x2F;www.propublica.org&#x2F;article&#x2F;claim-on-attacks-thwarted-by-nsa-spreads-despite-lack-of-evidence" rel="nofollow">https:&#x2F;&#x2F;www.propublica.org&#x2F;article&#x2F;claim-on-attacks-thwarted...</a><p>[2] <a href="https:&#x2F;&#x2F;digg.com&#x2F;2015&#x2F;why-mass-surveillance-cant-wont-and-never-has-stopped-a-terrorist" rel="nofollow">https:&#x2F;&#x2F;digg.com&#x2F;2015&#x2F;why-mass-surveillance-cant-wont-and-ne...</a><p>[3] <a href="https:&#x2F;&#x2F;www.ted.com&#x2F;talks&#x2F;trevor_aaronson_how_this_fbi_strategy_is_actually_creating_us_based_terrorists" rel="nofollow">https:&#x2F;&#x2F;www.ted.com&#x2F;talks&#x2F;trevor_aaronson_how_this_fbi_strat...</a>