It is distressing that the idea that we conquered Iraq with just three divisions has settled in to the conventional wisdom. Even Strategypage, in a good discussion of FCS recounting the value of our legacy heavy armor, repeats this so-called fact.
Let me correct this. We conquered Iraq in 2003 with the line equivalent of seven divisions. The hallmark of our invasion was the use of air power in place of the artillery and other support units that supported Desert Storm in 1991.
Without the need for traditional Army support units, we deployed 60 line battalions (infantry, armor, and cavalry), about equally divided between the Army and Marine Corps, plus 10 British line battalions.
Our single Marine "division" had 28 line battalions, or the equivalent of the line elements of 3 divisions. The Army had the line equivalent of 3 divisions. And the British added a division.
These seven division equivalents were completely in line with our post-Cold War thinking that we'd need 5 Army divisions and 1-2 Marine Expeditionary Forces to handle a Major Theater War against either North Korea or Iraq. The difference was that instead of the heavy presence of support units behind that front line strength, we relied on air power and more just-in-time logistics instead of ample separate artillery units and iron mountains of supplies.
Seven divisions--not three--defeated Saddam's military.