One final word on this: the issue here is not who was right and who was wrong, or even whether the overall weight of the evidence was sufficient to justify the war. It would have been perfectly reasonable for the White House to present all the evidence pro and con and then use that evidence to make the strongest possible case for war. But that's not what they did. Instead, they suppressed any evidence that might have thrown doubt on their arguments, making it impossible for the public to evaluate what they were saying. In fact, by abusing the classification process to keep these dissents secret, they even made it impossible for senators who knew the truth to say anything about it in public.Ok, in order to understand why this is important, stop and think about what an intelligence agency actually is, and what it does. It finds out secrets, and has to protect how it does it so that it can find out more. Because these things are secrets, an intelligence agency wants to keep it close to their chest.
The problem, though, is that there isn't one intelligence agency, there's TONS of them, and some are more powerful than others. They need other agencies' intelligence to be effective, and if one agency has the most intelligence, it gets to have the biggest say about the conditions under which it's shared and poses the greatest threat of cutting the whole thing off.
That most powerful agency is, naturally, the CIA (or NSA, depending on what kind of intelligence you're talking about). They're the most powerful, so they are the sellers in a buyer's market, and know it. A lot of the intelligence that's out there comes through them, and if other agencies have problems with the quality of the work or the analysis, they're going to shut the hell up about it in public, or the intelligence flow will get shut off. This isn't just a political problem; a shutoff can become a threat to national security for smaller countries dependent on the US. So they aren't going to disagree.
Thus, saying "everybody agrees" is irrelevant, because either the information they get is coming from the CIA/NSA already, or they're going to be publicly silent about their concerns for fear of repercussions at the hands of the Americans.
(The greatest exceptions to this rule are probably the British, who are masters as HUMINT (human intelligence), and the Israelis, where the Mossad's intelligence gathering abilities are legendary. Neither, however, had any reason to dissent, and may well have done the same thing to the Americans.)
Naturally, this isn't as big a problem as it sounds like, because intelligence agencies are certainly going to discuss and debate privately over intelligence, in order to serve their vital role of speaking truth to power. When they become politicized, however, and the "truth to power" role gets submerged in a pool of ends-justifies-means ideology, they become dangerous; if this happens concurrently with politicians moving in that direction, it becomes extremely dangerous. There is nothing, absolutely nothing, stopping a ideologically driven intelligence agency in the service of similarly ideologically driven politicians from being able to say whatever it wants to say, and if it's the big player, that includes other countries' intelligence agencies and the politicians they serve.
The media and public are near-helpless in this situation to do a damned thing, at least until the mistake becomes obvious to everyone. There will always be some sort of official "oversight" that supports the ideology in question (witness the intelligence committee and it's report), creating easy rhetorical cover for fellow-travellers, and national security is a goldmine of opportunities for draping oneself in the flag. The end result is what we have now.
Intelligence agencies are a necessary tool. We should never, ever forget, however, that they are a potentially dangerous one.