It was a pretty general statement yes. But I am well aware of how the military works, I signed up for the Marines and am shipping out to Paris Island next year.
But the fact is, the majority of our forces are infantry, or of an infantry-based MOS who can do just as much good in Afghanistan than Iraq.
In the Marines that is true, but in the Army which has the overwhelming majority of the United States ground combat units, that was not so at the start of the 2003 invasion of Iraq.
Of the 10 active US divisions in the United States Army in 2003, 6 were heavy armored or mechanized divisions, the armored divisions having 6 tank battalions for a total of 348 tanks and the mechanized divisions having 5 tank battalions for a total of 290 tanks. The number of infantry battalions in in the Armor divisions was 4, while it was 5 for the mechanized divisions. But these were infantry battalions were considered "heavy" as opposed to "light, because they used the 35 ton Bradley fighting vehicle. In addition, each division both armored and mechanized had 5 artillery battalions which consisted of heavy self-propelled 155 mm howitzers, the Palidin, and the MLRS rocket launch system.
So 60% of the active army at the time of the Iraq invasion was composed of heavy units that required sea lift for transportation instead of air lift in order to get to the theater of operations, and were obviously unsuited for operations in many parts of Afghanistan.
Each division has three combat brigades. All 3 armored calvery regiments in the army at the time of the invasion were "heavy armored" as well. Each is about the equilivant of a brigade.
So the Army's 33 combat brigades could be broken down as follows in terms of heavy vs light at the start of the 2003 invasion of Iraq:
Heavy: 21 combat brigades
Light: 12 combat brigades.
At the end of the Cold War in 1991 when the Army was larger, it had 62 combat brigades and the ratio of Heavy combat brigades to light was as follows:
Heavy: 44 combat brigades
Light: 18 combat brigades
The 38 combat brigades of the US Army National Guard had a similar ratio of heavy forces vs light forces at the time of the US invasion of Iraq in 2003.
The Active Marine corp has 3 MEF's each of which are the equilivant of about 3 or 4 US Army brigades. They are mainly light in that there is usually only one tank battalion per MEF, although the US Marine MEF used in the invasion o Iraq in 2003 was up armored with multiple tank battalions.
The Marine reserves only have 1 MEF.
So, combining the US Active Army brigades, US Army National Guard Brigades, and Marine/Marine Reserve equilivant you come out with the following at the time of the US invasion of Iraq:
Active Army: 33 Combat Brigades(21 heavy, 12 light)
Army National Guard: 38 combat brigades(24 heavy, 14 light)
Active Marine Corp: 3 MEF's or 12 combat brigades(roughly 1 heavy, 11 light)
Marine Reserive: 1 MEF or 4 combat brigades(Light)
Total ratio of heavy vs light at the time of the 2003 invasion:
Heavy combat brigades: 46
Light combat brigades: 41
So even when considering the lighter forces of the Marine Corp, the majority of US combat forces at the time of the 2003 invasion of Iraq were "heavy armored" and not ideally suited for operations in much of Afghanistan, without even considering the difficulty of moving them into the country.
In addition, the mix of forces on the ground in Kuwait that invaded Iraq in 2003 were heavy armored. The Marine MEF was up armored with more armor battalions(M1 tanks) and other heavy equipment. The US Army division was a mechanized division with 290 M1-A2 tanks that each weight nearly 70 tons and the infantry was equipped with the relatively heavy M2 bradley fighting vehicles coming in at 35 tons.
The other major heavy armor unit used in the invasion was the 1st British Armored division equipped with several hundred Challenger 2 tanks weighing over 60 tons and Warrior fighting vehicles for the infantry with a similar weight to the US Bradley.
The only truely light units in the invasion, the 101st Airborne division and a single brigade of the 82nd Airborne division were primarily held back in reserve and followed the armor units into Iraq. These were the
only major combat units without any heavy tanks or infantry combat vehicles that would be suited for military operations in any part of Afganistan.