Just as the Second World War could not end until Berlin had been taken, so Baghdad will have to fall before the United States and Britain can claim victory.
Saddam Hussein will have to be captured or killed, perhaps in the bunker he has constructed as a refuge reminiscent of the Chancellery in Berlin where Hitler remained until the end.
The comparison is not intended to compare Saddam Hussein directly to Adolf Hitler as a historical figure. It is intended to draw parallels between two diehard leaders facing a siege of their capital.
The emerging tactics to capture Baghdad therefore deserve the closest attention. If the city is not to open its gates, then the Americans will have to employ other means.
The humanitarian cost mounts
Maybe they will follow the British method seen around Basra - squeeze the city without besieging or conquering it. Maybe they will engage in the last-ditch tactic of street by street fighting.
Intention is to destroy the Republican Guard
Before that, they intend to destroy the Republican Guard forces arrayed to the south of Baghdad. The latest reports from the battlefield indicate that the fight has been taken as far as the town of Hindiya, 80 kilometres (50 miles) from Baghdad.
Hindiya, a town of 80,000 on the Euphrates, and with a bridge, is not far from the biblical city of Babylon which Saddam Hussein, claiming to be the spiritual descendent of the great Babylonian King Nebuchadnezzar, has partly rebuilt.
US troops in Hindiya
Associated Press reporters in Hindiya said that American soldiers from the 3rd Infantry Division (the heaviest of the three main US units heading north) had entered the town to find Iraqi forces in civilian clothes using guerrilla tactics to get close to US forces on foot and in vehicles.
As yet, the ground campaign is still only developing. The main emphasis has been on the use of airpower
Some prisoners were taken and they said they were from the Republican Guard's Nebuchadnezzar Division, an infantry division normally associated with Saddam Hussein's home town of Tikrit to the north of Baghdad.
Their presence so far south could indicate either that the Iraqis felt there was no threat from the north or that things were getting desperate enough in the south that reinforcements were needed.
But as yet, the ground campaign is still only developing. The main emphasis has been on the use of airpower. US and British aircraft, US Apache helicopters and long range artillery have been used to attack the Republican Guard defending Baghdad to the south.
Four Guard divisions under attack
There are reckoned to be four Guard divisions around the capital. The most powerful of these is the Medina armoured division which is deployed north of the holy city of Karbala.
The US Central Command claimed on Monday that 50% of the Medina division had been destroyed. Medina, incidentally, is another name carefully chosen by Saddam Hussein. Medina is the place where the prophet Mohammed rallied his forces.
The Gulf war of 1991 and the Kosovo war of 1999 taught us to be cautious about accepting such claims. It is unlikely that the Republican Guard forces are laid out ready to be destroyed as the Iraqi army was in Kuwait.
The American intention is clear enough - disrupt the Republican Guard and the way is open to Baghdad
Around Baghdad, in the fertile and flat land between the two rivers, there are small towns and villages, and clumps of trees where tanks and troops can be dispersed.
American intention clear
But the American intention is clear enough - disrupt the Republican Guard and the way is open to Baghdad, whatever the tactics which might have to be employed when they get there.
They want to stop Guard units from getting back into the capital and adding to its defences.
In the meantime, the air attacks on targets in the capital continue. Presumably the hope is that one of these will despatch Saddam Hussein.
Role of the 4th US division
The two other main American units are the 101st Airborne Division and the 1st US Marines. The 101st is deployed around Karbala and has a sizeable force of helicopters but no heavy armour. It is being used to secure the supply lines right now but could be used as a mobile strike force.
The comparative lack of US troops has been noted. The Pentagon has stressed that numbers matter less these days than capacity. But will the US wait until the 4th Infantry Division, expected to be operational in about two weeks, enters the battle? Probably not, if General Tommy Franks decides that tactical advantage calls for quick exploitation.
Marines on right flank
The marines, according to New York Times reporter Dexter Filkins embedded with the unit, have advanced to Hilla, also not far from Babylon. He quoted the colonel as saying "We're in bad guy country. I like it." The marine advance has just taken them 40 miles north in one lunge after a "furious battle."
Filkins says that the marines appear to be forming the right hand flank of the main American force which is concentrating around Karbala.
There has been talk among strategists that the main US thrust to Baghdad might come through the so-called Karbala gap between the Euphrates and a major lake to the west. This would bring them to the south west of the capital.
The real problems are likely to start when they get there.