By Jonathan Marcus BBC defence correspondent

The presentation by US Secretary of State Colin Powell at the United Nations Security Council did more than just provide evidence to back Washington's assertion that Iraq is systematically thwarting the weapons inspectors.

Detainees in Guantanamo Bay may have provided intelligence

In an unprecedented way, it drew back the veil on the vast array of America's intelligence gathering activities.

It showed the types of information that can be collected, ranging from intercepted radio signals and telephone conversations to satellite photos.

It also showed the way it is mixed together with material from the de-briefing of defectors and the interrogation of al-Qaeda suspects.

Many sources

Much of this material, we are told, was released only after a serious struggle within the US administration and, in particular, between the US State Department and the wider intelligence community.

All collected information is considered by intelligence analysts

Intelligence officers rarely like to reveal anything, fearing that publicity given to their product will inevitably compromise some of the sources from which they obtained their raw materials.

For intelligence is in this sense "a product"; it takes inputs from many sources.

US satellites and other technical means are probably monitoring hundreds of thousands of Iraqi communications.

These have to be filtered to sort the mundane from the useful.

But even mundane information - when put together with other fragments of the jigsaw - can suggest a particular picture.

Putting that picture together is the work of intelligence analysts.

No 'legal' evidence

Satellite photos can be scrutinised by experts, individuals' movements can be plotted from their communications; flows of money can be pursued.

It may seem paradoxical, but matters of war and peace are often determined on standards of evidence that fall far short of legal proof

But ultimately, as Mr Powell's presentation showed only too well, intelligence is not evidence in the legal sense.

Few intelligence cases would necessarily stand up in a court of law.

Intelligence gatherers are not like policemen who investigate after a crime has taken place.

They are constantly monitoring a whole variety of sources, searching often for that proverbial needle in a haystack.

They have to face active efforts by their target to obfuscate and conceal.

High stakes

What they eventually come up with is, inevitably, only part of the picture.

It may seem paradoxical, but matters of war and peace are often determined on standards of evidence that fall far short of legal proof.

The stakes in national security are high.

And often governments have to act on the intelligence evidence that they have, rather than on what they might like to have.