There must be times when those who serve us secretly wish that the word "intelligence" had never been attached to their calling. It tends to raise expectations. It also tends, sooner or later, to involve an insult to the average person's share of the commodity mentioned in the job description.

The Educated Guesswork in a Defined Political Context Service would probably be more congenial. The Central Speculation Agency could easily provide jobs, earpieces and mirrored sunglasses for life.

Most of us would never notice the difference. No harm done, as it were. After Iraq, many would count that as a decent policy objective.

True, some politicians would probably grumble at being deprived of the unimpeachable evidence on which, traditionally, wars depend. But Tony Blair and George Bush have already shown us a way around that obstacle. With a merely notional threat and a couple of impressive-looking self-assembly dossiers, your major conflict is just a cut-and-paste away. Now that's intelligence.

It becomes a little embarrassing for the professionals, however.

One little-noticed consequence of Iraq had nothing to do with the near-universal conviction that two statesmen had fibbed through their molars with the aid of manufactured intelligence. Instead, there was the amusing conclusion made available to enemies and potential enemies of Britain and America.

Translate into the language of your choice. These guys don't know anything, the rogues must have said. They haven't a clue. They even botched the lies, but the point is: they didn't really know diddly about Saddam, his intentions, or his weapons. We can relax.

Voters in the democracies, if they care, are drawn to the same inference. Either numerous lies were told, or that "catastrophic failure of intelligence" exposed the entirely spurious basis on which foreign policy was being conducted. Bush and Blair did not say "we believe"

or "we fear" when talking about Saddam's destructive potential. They said "we know".

After that, Iran was always going to be a hard sell. A weird kind of credit is due to the White House, therefore, for pressing ahead with apocalyptic visions of a nuclear-armed Mahmoud Ahmadinejad wiping Israel from the face of the Earth while creating havoc in the oil markets. Mr Bush, in particular, showed what he was made of by applying precisely the same rhetoric to Iran as he had expended on Iraq. Couldn't fail twice, could it?

Where Iran was concerned, the President knew. The President knew, and said frankly, that Tehran was working furiously to acquire the bomb and threaten all and sundry. Mr Bush was firm and unambiguous, just like the intelligence, or so we were told, on which he relied. In fact, not so very many weeks ago the world's most powerful man spoke of Iran and the Third World War in the same breath. He had reached the last page in the Book of Scary.

The President's chief prop was the National Intelligence Estimate, a joint report by America's 16 separate security agencies on the threats facing that country and the west in general. Just two years ago, this spooks' compendium asserted with absolute conviction that Iran was "determined to develop nuclear weapons". This week, as sheepishly as 16 hugely-powerful multi-billion dollar agencies can, the authors of the NIE changed their tune.

Had Iran's determination been beyond doubt? Was there no room for ambiguity, uncertainty, or equivocation? In fact, the NIE was unambiguous this week in asserting the opposite of what it had said with confidence in 2005. Tehran's nuclear status-envy might be intact, but the report, Iran: Nuclear Intentions and Capabilities, concluded: "We judge with high confidence that in fall 2003 Tehran halted its nuclear weapons programme."

Just like that. Mr Bush could put away his big stick. The Europeans could begin attempting to persuade the sceptical Russians and Chinese that a new, tighter round of sanctions against Iran was still necessary. Uranium is still being enriched there - not a reassuring sign - but the US agencies now admit that the Iranians will not possess weapons-grade material until after 2015.

That does not count as a guarantee of peaceful intent, of course. Unlike its nuclear programme, Iran's sponsorship of terrorists and insurgents around the Middle East, along with its loathing of America, shows no sign of abating. Last month the International Atomic Energy Agency reported that its access to the Iranian nuclear effort was still being restricted. Israel has meanwhile reacted sceptically to the NIE document, arguing that Tehran, having halted work briefly, has resumed its pursuit of the bomb.

Some facts remain, nevertheless, amid all the "intelligence". One is that Israel already has nuclear weapons, whereas Iran, according to the agents of its sternest critic, does not. Pakistan - and how clever does this now seem? - is also an atomic power, thanks to America's connivance. Definitions of threats to peace and security appear to be a matter of taste.

Almost as worrying is the sheer speed with which yesterday's indisputable truth has become today's tiny regrettable error in reporting. Allegedly, new "intelligence" has become available to the authors of the NIE. Perhaps so. But what became of 2005's gospel truth? Who erred, lied or simply moulded scraps of fact, gossip and suspicion into the certainties that allowed Bush to talk about a Third World War?

Iraq, at the risk of stating the obvious, proved that people die when intelligence agencies "judge with high confidence" while misreading and misrepresenting reality. The emerging history of the Iraq war tells us that "intelligence" was bent and twisted to fit a geopolitical ambition. But even if you allow the benefit of every doubt, can you really trust politicians prepared to rely on sources so unreliable?

Washington's tribal politics has no doubt played its part in the NIE volte face. After Iraq, the intelligence agencies were showered with blame, either for misleading poor, trusting politicians, or for being prepared to manufacture evidence on demand. This time, clearly, the spooks are having none of it. If the White House persists with plans for a military strike against Iran, the agencies do not intend to be held responsible.

Besides, Mr Bush has barely a year left for Middle East adventures. His successor will almost certainly be a Democrat disinclined to endorse another farrago. The intelligence people are intelligent enough (probably) at least to understand how Washington operates. Now, for once, they can plausibly deny something that they do not, in fact, believe.

Perhaps that is all there is to it.

A mistake has been recognised. Another catastrophic intervention has been postponed, and probably averted: why complain? First, because the politicians will not be deterred from mistaking convenient guesses for evidence. Secondly, because actually reliable intelligence remains a requirement in a dangerous world. Finally, because too few of us have yet learned to be careful about the things we choose to believe.

There was a time when wars and the occasions for war were observable, demonstrable and explicable. Now evidence is invented to justify fantasies. They should offer an estimate of the intelligence of those who still fall for that.