By Jonathan Marcus BBC defence correspondent

Starting with key Congressional leaders, US President George W Bush has started - some would say belatedly - to make the case for regime change in Iraq.

Behind the scenes a range of military preparations are already under way.

Some represent long-standing prudent house-keeping and may not relate to any specific US war-plan.

The armchair generals and pundits have had a field day and they are far from finished yet

But a whole variety of factors - the call-up of certain types of reservists, the chartering of commercial shipping, the reported movement of pre-positioned equipment both to and within the Gulf region - all suggests that if war comes, it could begin with far fewer signs of obvious preparation than many people have argued.

There has been much speculation in the media - often fuelled by carefully placed leaks from the Pentagon - about the game-plan for a campaign to topple Iraqi President Saddam Hussein.

Much of this relates to internal US military concerns.

But it is also part of a wider campaign to confirm to America's friends, as well as to the Iraqis, that the Bush administration is serious about military action.

There has been the outside-in-plan, a kind of re-run of the war to liberate Kuwait but with Baghdad as the ultimate destination.

There has been the inside-out plan, which is the idea of somehow isolating and destroying the Iraqi command centres in the capital, sending the Iraqi military into chaos and then destroying it piece by piece.

Afghan model

Some have spoken of a "Gulf War light" pointing to the overwhelming (and perhaps unnecessary) level of force gathered for Operation Desert Storm.

Others have talked about possible "Afghan models" using local opponents of Saddam backed by US Special Forces.

The armchair generals and pundits have had a field day and they are far from finished yet.

President Bush says he will consult his allies

Any eventual US operation may involve elements of all these plans.

I am not going to speculate here, but some of the key ingredients of any operation can be predicted.

Firstly, just as in Kosovo and Desert Storm there will be an extensive and intense air campaign.

Its purpose will be to destroy Iraqi air defences and then wipe out command centres and so on.

Elite troops targeted

It will also seek to destroy what weapons of mass destruction infrastructure can be identified.

The air war will also try to force Iraq's best armoured formations - the Republican Guard - to deploy.

It is hoped many Iraqi soldiers will refuse to fight

Once deployed, it too would be hammered from the air.

Information operations will play a significant part in the US calculations.

It will be trying to destroy the props of Saddam Hussein's regime and to convince ordinary Iraqis that Saddam's days are numbered.

The hope will probably be that much of the ordinary Iraqi army will not put up a serious fight.

So much the better, according to US calculations, because the Iraqi army may be needed after Saddam is toppled to maintain order and keep the territorial integrity of a rudderless Iraqi state.

Lightning speed

Above all else the Americans will be seeking to capitalise on their extraordinary advantages in technology.

It is not so much the machinery and weaponry themselves but the way they are used.

Tank battles in Operation Desert Storm underlined the ability of America's modern war machine to deal out destructive power at a speed and intensity that completely overwhelmed the Iraqi units facing them.

Saddam says Iraqis are ready to defend themselves

All sorts of disasters and pitfalls could face America's military planners.

Iraq is a very large country. The support of some of America's allies in the region will be required however tacit their approval, otherwise US operations risk being funnelled into narrow and predictable manoeuvres.

Aircraft carriers alone cannot sustain the tempo of sorties needed in such a campaign even if supported by long-range bombers operating from beyond the Gulf region.

Plans are clearly being drawn up.

But for the moment the battle is still very much at the political level as the Bush team tries to achieve consensus at home, without which any war is probably impossible.