Without our continued presence in Iraq, Iran could entertain the notion of invading Iraq. When the balance of power had shifted to Iraq after the Iranian Revolution, Iraq pounced on Iran. Iran might return the favor while the balance (without American forces) shifts to Iran until Iraq can build up their conventional fighting capabilities. AEI looks at the balance of conventional forces.
I did my own back-of-the-envelope calculations of ground forces. I agree.
I took open source information about the Iraqi and Iranian armed forces and used my own guesstimates of the relative value of Iraqi and Iranian ground units, in a manner similar to what I did before the Iraq War. Part of it was anchored on estimates given to United States combat brigades and then adjusting Iraqi and Iranian units in relation to our units. I assumed the vast majority of Iraqi units are light infantry forces not yet capable of fighting conventional battles with any strength and that even the efforts at forming conventional heavy units has not progressed enough to reach full Iraqi effectiveness in that area. So I came up with two sets of numbers for the Iraqis--now and after converting to conventional capabilities. I also estimated notional deployments of Iraqi and Iranian ground units to estimate how much power each could bring to the border.
I estimate that Iraq's ground forces have a combat potential of 44 points. Iran has 140 points. This is more than a 3:1 advantage overall. That's a decisive margin.
Iraq could deploy a higher portion of their forces on their eastern border in the center (forward of Baghdad) and south (forward of Basra). I'll call it as 34 points there with 10 points in the far north and west. Iran would have 9 points in their northwest Kurdish region, 25 close to Tehran, 9 on the Afghan border, 9 on the Pakistan border, and 9 on their coast. That leaves 79 points for the main fronts in the center and south.
So Iran would have 79 ground points to Iraq's 34. With more than 2:1 at this scale, that is decisive. I know that a lot of writers would look at those numbers and chant the "3:1 rule" and say Iraq is safe, but that ratio applies to lower levels of analysis (and then considers the benefits of terrain and other factors on the "points" of the basic units). One side in a campaign with even a 1:1 equality overall could strip forces along the front to reduce local balances to 2:3 while massing combat power at the point of attack up to 6:1 or higher. Recall that the German blitzkrieg of France in 1940 had the Germans on the short end of tank numbers by a considerable margin and were pretty equal in troop strength. Yet the Germans concentrated the bulk of their tanks at the allied weak point and overwhelmed the spread-out Allied tank strength which could not mass to respond to the breakthrough. Campaign over.
So Iran, starting with a sizable overall strength advantage, could hammer the Iraqis along a small number of axes of advance. Iran would likely have trouble supplying such an attack for long, so their strength would dwindle in absolute terms. The question will be whether the offensive destroys Iraqi strength enough to maintain the Iranian edge and allow the offensive to continue. American air power would intervene in short order, but unless American spotters are on the ground it will be less effective than it could be.
So in the short run, Iran has the advantage. Will they exploit it to invade or even just use the threat of invasion to pressure Iraq to comply with Iranian demands?
American forces would play an important role in this short run. If we keep 3 Army brigades in Iraq, as I want, we'd add 27 points (I assume 9 per Army brigade). Toss in an afloat Marine Expedionary Unit (4 points) that could be landed quickly, and maybe another Army brigade in Kuwait, and we'd add 40 points to Iraq's total. Add in the equipment of two more Army brigades in the region that could be quickly manned by troops flown in and a Marine Expeditionary Brigade with equipment afloat (15 points), and we'd be able to quickly add 33 more points, for a total of 73 American ground points. Now the Iranians face a deficit of 79 to 107 US/Iraqi points. Add in the effects of American air power, which could double the effectiveness of American ground power, and the war would quickly become a matter of Iran defending Khuzestan (or Arabistan as the Iraqis probably still like to call it).
Granted, these numbers are just educated guesses on my part, but that's what the process would look like with real numbers based on deeper knowledge of the forces involved. Even with my limitations, clearly, with even a small American force inside Iraq with more on call, Iran wouldn't even try to invade and could not pressure Iraqi officials with that threat.
In time over a period of years (I don't know how many. Five years under "peacetime" conditions? Although Iraq's air force probably needs a decade to train, I think I read.), as Iraqi forces are equipped and trained for conventional warfare, I guess that Iraq's ground combat numbers go up to 114 points. If 25 points are committed to the north and west, that leaves 89 points to confront 79 available Iranian points on the main fronts. At this point, even the ability to simply quickly reinforce Iraq with 3 Army brigades (1 in Kuwait and 2 from prepositioned equipment, for 27 points) and a MEU and MEB (19 more points), plus American air power, will deter Iran from thinking about war. Iraq could also use their historically decent engineering capabilities to build fortificatons, mine fields, and obstacles along the border as force multipliers. I'll assume the use of poison gas for that purpose is right out, these days.
Of course, should Iran escape sanctions, they could probably up their combat points by a good 50% by just getting adequate spare parts and supplies to put what they have in action--to 210 points--but a relatively small amount of nearby American ground and air power would again provide the margin of superiority to deter Iran. Without our presence, Iraq would need to go beyond their existing force structure in that case.
AEI also notes an advantage of American forces providing the necessary edge against Iran--it means Iraq does not need to build up its own defenses against Iran quite as much. Because if Iraq builds up its military to hold off Iran, it necessarily builds up a conventional military capable of offensive action. Just remember that Iraq built up its military to fight Iran and when that war ended in 1988, used that large military to quickly overrun Kuwait in 1990.
All in all, Iraq and the region need us to stay in Iraq to preserve the peace. And this is in addition to our important role in preserving and nurturing Iraqi democracy and rule of law.