By Jonathan Marcus BBC defence correspondent

When US Defence Secretary Donald Rumsfeld asked the Pentagon top-brass to come up with an invasion plan for Iraq, they reportedly presented something similar to the 1991 Desert Storm operation.

This involved huge numbers of troops and months of costly build-up and preparation, followed by weeks of grinding air attacks against Iraqi positions.

Ground troops will be essential

The story may be a little self-serving, put out by the defence secretary's own staff, but Mr Rumsfeld, we are told, took one look and sent them back to the drawing board.

What has emerged is a radically different plan, depending upon a much smaller ground force and a shorter, but more concentrated air campaign.

The air and ground aspects of the operation are likely to be mixed together. At any rate there will not be anything like the 39 days or so of air strikes of Desert Storm.

Special forces

The preliminaries in this conflict have already begun. And it is not just the obvious US and UK military build-up in the Gulf.

There are also reports of US Special Forces operating in northern Iraq.

There are unconfirmed reports of Western special forces taking an active interest in the so-called "Scud boxes" in western Iraq, from which Iraq might try to direct missiles against Israel.

And the increasing frequency of US and UK air attacks against air defences in the northern and southern "no-fly zones" is not just a matter of intensity.

Targets are being carefully chosen to slowly degrade Iraq's military capabilities.

Rumsfeld's plans promote a new form of warfare

If and when war comes it is going to be swifter and more concentrated than many people imagine.

And in many ways it will be the first great test of a very new doctrine of warfare.

The aim is not so much to destroy Iraq's armed forces. They will only be destroyed if they fight back - although units like the Republican Guard, who are seen as being close to the regime, will be targeted.

The aim is to use military force to create "an effect" i.e. the removal of Saddam's regime. The aim will be to do this with as little long-term damage to the civilian infrastructure as possible.

New-style warfare

This will be the so-called revolution in military affairs in action - the coming together of the dramatically increased accuracy of weaponry, new intelligence gathering systems, and new ways of spreading information.

The transition to this new form of warfare is by no means complete.

But the war in Afghanistan heralded some of these developments. It has highlighted how different this new war in the Gulf might be from the 1991 conflict.

All the indications from the Pentagon are that this war will begin with intense attacks on air defences and command centres.

The aim will be to isolate the regime and to break down the chain of command.

While the eventual outcome of any conflict is not in doubt, all sorts of thing could still go wrong

Once Iraq's defences are knocked off balance the aim will be to keep them that way. Pace and firepower will characterise this conflict.

Special forces will play an important role, not least in trying to track and destroy any scud missiles and to secure or isolate chemical weapons stores and to protect vulnerable oil installations from sabotage.

Clearly, while the eventual outcome of any conflict is not in doubt, all sorts of thing could still go wrong.

Mr Rumsfeld's central task has been to transform the US military.

It is very much a work in progress and he has met considerable resistance along the way.

But if this war turns out the way Mr Rumsfeld hopes, some of his internal critics will have to think again.