Does it depends on what you mean by "know"? Stephen Walt throws out some possibilities on the subject of spying and foreign policy making.

It is hard to believe we are, given that America's foreign policy record since the end of the Cold War is mostly one of failure. And that leads me to suspect that one of two things is true. Either 1) the NSA is good at collecting gazilla-bytes of stuff but not very good at deciding what to collect or figuring out what it means, or 2) the rest of our foreign policy establishment is not very good at taking advantage of the information the NSA has worked so hard to acquire. In other words, either the NSA is not worth the money we're paying for it, or the rest of our foreign policy establishment is less competent than we thought. To be frank, I'm not sure which possibility I prefer.

As far as we "know," preventing terrorist attacks (the reasons NSA has a huge budget) has not been so glowing: the Boston marathon attackers were on the radar screens of the FBI and the Russian intelligence services. The older brother was on jihadi web-sites and on the phone to Dagestan. Did the NSA have info that could have raised a red flag at the FBI?