What about those "intelligence warnings" ??

[changemonger]

There's something about almost every terrorists' attack that is rarely discussed and that I find a bit disturbing : the intelligence warnings. As it turned out after the Mumbai attacks, India has been warned, just like the US before 9/11, the UK before the London bombings,... Isn't the primary role of intelligence agencies to AVOID those attacks and how do they know about them before they happen anyway ?. I know that the ETA in spain calls the authorities a few minutes before the attacks to warn them , but in the case of India,London or 9/11 there were ,as reported, warnings weeks or maybe months before the attacks . Why would the intelligence know about impending attacks and do nothing significant to stop them from happening ? I don't get it, do you?