Human Intelligence, CIA
Signal Intelligence, NSA
The English have been masters at the spy trade for centuries. In WWII, the United States felt that it should get into the act and turned to the English for guidance.
With their tutelage, the CIA became a formidable tool against the Soviet threat throughout the cold war. We had clearly defined enemies with clearly defined borders. Gathering intelligence became a methodical science... then, once the Soviet Union collapsed, the clearly defined enemies with clearly defined borders went with it.
The growth of the internet created an atmosphere wherein information and 'intelligence' became a commodity. Then the emergence of an enemy that is not only difficult, if not impossible, to clearly define but who also operates entirely without borders. The polar opposite from what the CIA were trained to do.
Not only has this rule-set reset turned the CIA upside-down, it has rendered it all but useless. The UK isn't doing much better either. The problem is that western society itself is at odds with the rules required to make an effective spy agency. Our open government(s), free access to information, laws against spying on citizens and so forth are what both protect our civil liberties as well as create the environment in which our enemies can plot against us.
The CIA knew about al Qaeda operators operating in the USA prior to 9/11, yet did nothing to notify the FBI. This is because of the opposing nature of each agency. The CIA finds a criminal and wants to string them along to see what intelligence they can uncover by monitoring them. When the FBI finds a criminal, they want to string them up. From the CIA perspective, the FBI sure knows how to screw up an investigation and destroy your intelligence network.
The CIA is now dysfunctional to the point of uselessness. In fact, there isn't a single effective spy agency in the western world. The current battle we're fighting and the enemy we face is one that cannot be defeated by military might, it is a war that MUST be fought using intelligence.
So, the administration turned to the only other agency with experience in gathering and monitoring enemies. It also happens that this agency is experts at SigInt, as opposed to the HumInt. The problem is that the NSA is forbidden by law from spying on American Citizens, UNLESS they are monitoring overseas communications. This exception has always been allowed, no warrant necessary. There is no law that states that I have the constitutional right to conspire with enemies overseas.
No other nation even comes close to the SigInt capabilities of the NSA...