By Tom Carver BBC Washington correspondent

The American military is expecting a war in Iraq. President George W Bush has made it clear he wants a regime change, and no one expects Saddam Hussein to bow out voluntarily.

But the how and when are still being worked out.

Having thousands of American troops and tanks in reserve on Iraq's borders would put tremendous psychological pressure on the regime

Michael O'Hanlon, Brookings Institution

Whether this is done with the blessing of the United Nations or by the US on its own will affect the shape and timing of an attack. But here is a possible scenario.

Before President Bush gives any obvious green light, Special Forces are dropped into Iraq to locate Scud ballistic missiles and as many chemical and biological weapons as possible.

"They've got to get into the 'Scud Box' to stop Saddam launching the missiles against Israel," one former Special Operations commander, Colonel Tim Eades, told BBC Online.

The "Scud Box" is the area of western Iraq from where a Scud could reach Israel.

Israeli Prime Minister Ariel Sharon has said any attack on his country would meet with massive retaliation, and America is determined not to let "regime change" spill into a wider Middle East war.

Lessons learnt

The next stage is that a large pre-positioning force of at least 50,000 troops moves as close as possible to Iraq.

US marines would play a key role in a ground assault

They will travel, mainly by ship, to Kuwait, Qatar and possibly Saudi Arabia, accompanied by their heavy armour.

Units such as the 10th Mountain Division, the 101st Airborne Division and the 2nd Marine Expeditionary Force are likely to be at the forefront.

While they are getting into place, a rolling air campaign begins. Like the Gulf War, this will try to sever the Iraqi military's command and control, cutting Saddam Hussein off from his commanders in the field.

This time it will be harder. The Iraqi leader learnt from the Gulf War not to station his forces in the open desert where they can be picked off from the air.

So he will keep his toughest, best equipped units close to hand around the two strategic areas of Baghdad and Basra, hidden amongst the civilian population.

The third stage is what is known as the "inside out" option, where 10-20,000 airborne troops head straight to Baghdad to decapitate the Iraqi regime.

Messy fight?

How easy this will be remains a matter of fierce debate. Will Saddam Hussein unleash his chemical weapons or could he be persuaded not to by the offer of some kind of asylum outside Iraq? How quickly will the Iraqi units crumble?

Missile strikes are only one of the options

Michael O'Hanlon, defence analyst at the Brookings Institution, says: "Having thousands of American troops and tanks in reserve on Iraq's borders would put tremendous psychological pressure on the regime. It'll get the message that it cannot win."

Optimistic military planners hope for a quick operation with under 1,000 US casualties.

But if we start seeing lines of American tanks streaming across the desert, as we did in Desert Storm, then we will know that the "inside out" option has not worked and that they are having to call in the heavy boys.

No one thinks America will lose. And the US military seems to be confident that it can run this at the same time as the rest of its war on terrorism. But the fighting could be messy.