The Tigris and Euphrates river valleys, in ancient times one of the cradles of civilisation, are now the cockpit for a massive sweep by US armoured columns who are moving forward at a remarkable speed.
It is an area whose geography is determining the nature of this campaign.
Imagine the Euphrates River as a line running in a north-westerly direction from Basra towards Baghdad.
Advance elements of the US army have already engaged Iraqi forces outside An Najaf, only 160km from the Iraqi capital.
Further downstream, between the towns of Samawah and Nasiriya, US troops have captured some crossing points and appear poised to advance in a second thrust up the northern side of the Euphrates.
Classic manoeuvre warfare
A third thrust is moving northwards from the Basra area up the Tigris river valley and clashes have been reported south of a town called al-Qurnah.
These towns are simply reference points on the map. They are not military objectives and for the most part advancing US units are simply sweeping on past them. What matters are the bridges.
American forces have progressed halfway to Baghdad
This is classic manoeuvre warfare, but there is still Iraqi resistance. US troops are beginning to engage significant Iraqi units, although initial reports that Republican Guard formations have entered combat do not seem to be correct.
The fighting in Nasiriya matters if it threatens US control of the river bridges. The combat in the port of Umm Qasr has little relevance for the main advance but it will hold up efforts to clear the port of any booby traps and thus will delay its use for the importation of humanitarian supplies.