By Gary Eason
BBC News Online
The US 3rd Infantry Division is closing in on Baghdad
As US forces close on Baghdad there are signs that the defending Iraqi units have fragmented - or been re-deployed.
The situation is highly fluid. How much this is by design - with both sides using feints in an effort to achieve surprise - is not clear.
Advancing on the left, at the end of a lengthy supply chain, are elements of the US V Corps - primarily the 3rd Infantry Division, a mechanised force with Bradley fighting vehicles and about 200 Abrams tanks.
Its Aviation Brigade includes Kiowa Warrior armed reconnaissance helicopters.
It is also supported by Apache attack helicopters from the 82nd and 101st Airborne Divisions.
In total, these troops number between 17,000 and 20,000.
On the right but further away are US marines - the 1st Marine Division plus, it is reported, the 15th and the 24th "expeditionary units" (MEUs) and the 2nd expeditionary brigade (MEB).
US Marines and the Republican Guard
Both considered "elite" forces in respective countries
Marines' standard weapon: M16A2 assault rifle
Republican Guard's standard weapon: Variant of AK assault rifle
Marine recruits face 13 weeks "boot camp" training
Fuel and ammunition shortages have led to a lack of training for Iraqi soldiers since 1991
In all, the marines probably have between 20,000 and 30,000 combat troops in Iraq, plus about 120 main battle tanks and a number of amphibious assault vehicles.
These forces can call on considerable air support.
The marines alone have more than 380 combat aircraft at their disposal but there are also US Navy and US Air Force and Royal Air Force attack planes flying constant ground support missions.
Very importantly but hard to quantify, there are US, British and Australian special forces inside Iraq.
They are said to have a free rein in the western desert - in the absence of media coverage - and have seized, for example, the huge Haditha Dam on the Euphrates River north-west of Baghdad, to prevent it from flooding the valley below.
Overhead - apart from the strike aircraft and helicopters - there are coalition surveillance drones and reconnaissance planes providing commanders with electronic and photographic intelligence on what is happening on the battlefield.
On the other side, it appears the Iraqis have split up and re-combined elements of different units.
Defender in Baghdad - but where are the big guns?
There were reported to be four of the six Republican Guard divisions to the south of Baghdad - nominally more than 40,000 men in total.
On the left, up against the US 3rd Infantry was the Medina division with up to 20,000 men and some 270 tanks, reinforced by elements of the Nebuchadnezzar division, which has about 10,000 men and 190 tanks and is normally based around Tikrit in the north.
On the right facing the US marines was the 2nd Baghdad Infantry Division, with again about 10,000 men and long-range artillery and rocket launchers.
Coalition commanders on Wednesday described the Medina and Baghdad divisions as no longer "credible forces" following a week or more of relentless air and artillery attacks.
Unusually they were said to have been reinforced by regular army units, which US officials are taking as a sign of weakness.
To the west of Baghdad is the Hamurabi Mechanised Division of the Republican Guard, potentially threatening the US 3rd Infantry Division.
Marines at Numaniyah in central Iraq
On the east of Baghdad is the Nida Armoured Division, which reports say might have been split up.
In theory the Republican Guard is used to form a buffer between regular army units in the south of Iraq and Baghdad.
It is not usually allowed into the capital, which is the preserve of the Special Republican Guard (SRG) - a separate organisation.
The SRG, headed by Saddam Hussein's son Qusay, is a hard core protecting the Iraqi leadership.
It is reported to have anywhere between about 12,000 and 25,000 men with motorised infantry, tanks and air defence units.
The Iraqi air force - and army air force - have not been heard of since the war began.
A US spokesman said the reason for their staying on the ground was simple: "If you fly, you die."
Coalition bombing has been focused on wrecking the Iraqi military's communications infrastructure, to deny those in charge an awareness of what is happening and the ability to control it.
The big mystery
On Thursday, the US 3rd Infantry Division was said to have closed to within a few miles of the Iraqi capital, pushing up from the south-west.
Surprise has been expressed at the lack of opposition met by US troops
But reporters with it say it has not been encountering any sizeable resistance.
"The numbers against the Americans were not that significant," said the BBC's Gavin Hewitt.
The Iraqis did seem to have been surprised by the speed of the US advance, he added - but he thought it premature to say that the Republican Guard divisions expected to defend the capital had been overcome.
He had not seen large numbers of destroyed vehicles or prisoners.
BBC correspondent Rageh Omaar, in Baghdad, said the surprise there was that it still seemed to be an open city - with no checkpoints, troops or tanks on the main arterial routes.
Moving around the city he said the picture was of many different forces - regular army, paramilitaries and militias in small units all over the place, but no armour.
"It does not have the appearance at all of a militarised city.
"Something for me just simply does not add up," he said.
"Where are the defenders? Nobody knows."