How would a hypothetical war between the US and Iran play out?
Originally Answered: How would a US war with Iran play out?
Any US invasion of Iran would likely start with a several months long build up of American troops in the Middle East with the need to transfer multiple divisions of ground troops as well as significant air and naval assets. Let’s assume for the sake of this question, the navy will commit three aircraft carrier battle groups or approximately 80ish aircraft each while the air force deploys nearly a thousand aircraft to Iraq, Kuwait, and Saudi Arabia. I suspect the numbers could be even higher, but I don’t particularly think that there will be a significant combat difference with say 1500 aircraft deployed versus 1300 aircraft, given that US air power will have near total control of Iran’s airspace from day one.
In regards to ground troops, you’d probably see the deployment of some combination of two Marine divisions, First Infantry (the Big Red One), First Air Calvary, 101st Airborne, First Armored Division, and 10th Mountain Division, with a significant deployment of special forces and specialized light infantry units such as Rangers. Significant division sized formations such as 2nd Infantry would likely remain in Korea to face North Korea and potentially the 3rd Marines would remain in Okinawa to defend against a potential conflict with China. Approximately a quarter of a million US Army troops and support personnel were used during the 2003 invasion of Iraq and presumably a similar number of troops could be deployed for an invasion of Iran.
Let’s say for the sake of this question, that the 1st and 2nd Marines, First and Third Infantry, 101st Airborne, First Armored, and 1st Calvary Divisions are the deployed units. Seven divisions with support elements. With deployed support personnel from other services for aircraft and the naval force, there would probably be upwards of 500,000 troops in theater with most of the best US combat forces. The US would have to call up reserve units for follow up and occupation duties assuming the war goes well or reinforcements should these forces fail to strike a knock out blow. The Saudis would likely try to assemble some troops, let’s call them a division sized formation of dubious quality, and some aircraft to aid the invasion.
This invasion force would nearly certainly have to come from Iraq, Kuwait, and Saudi Arabia. The Turks sat out the second Iraq war and they have even less issues with Iran than they had with Saddam, while Pakistan is nearly certainly going to avoid a conflict with Iran given their focus on a war with India at any moment. It is possible to launch an invasion from Afghanistan, but the mountainous terrain and long supply lies through Pakistan make that unappealing. Starting in Iraq and marching directly across the border with naval landings in the Persian Gulf would be the least complicated invasion route and logistically the easiest to support with US naval power.
As an initial matter, the biggest problem between Iraq and Iran, is that Iran is a much more formidable country than Iraq. In population, more than twice as big, while in land size, more than three times bigger, and GPD approximately five times bigger than Iraq. Iran has about 850,000 troops, reserve and regular, in its military with a 120,000 Revolutionary Guards units probably being the best equipped and most motivated. They would nearly certainly deploy every man they had and seek to use militias and fresh conscripts to reinforce their units. Let’s say a million total initially with rapid conscription and training to try to bring in more bodies to the front line.
Also, unlike Iraq, Iran is majority Persian Shiite nation without the internal conflicts that made Iraq much more difficult to unite against a common enemy. There is unlikely to be widespread breakdown of Iranian military units or uprisings by minorities, with the possible exception of the Kurds in the north west, that complicated Iraq’s defense in 2003. In addition, Iran hasn’t been under punishing international arms sanctions depriving them of decent anti-tank and anti-ship weapons. Based on what Hezbollah has used against Israel, we have a pretty good idea of what antitank and anti-ship weapons that Iran has access to. While they may not be terribly effective against Abrams tanks, Bradleys and Strykers and trucks would be much more vulnerable to their anti-tank weapons and IEDs.
Based on what Hezbollah did in 2006 during their fighting with Israel and based on confrontations between Shiite militias in Iraq and US forces, we can also predict that Iran would deploy their forces in a similar defense in depth with a heavy reliance on stay behind and detached light infantry to attack and disrupt follow up units with anti-tank missiles and IEDs. Avoiding major concentrations of infantry and armor out in the open would prevent them from being cutting to pieces from the air. They would likely plan their main defenses in urban areas and spread out their armor in urban areas, however, the main defensive line would like be the Zagros mountains.
As evident from this topographic map of Iran, there is a rather problematic mountain range between the invasion jumping off points of Iraq and Tehran, where an invasion force eventually needs to take. Fighting through mountain passes naturally creates a bottleneck where the defenders can make life difficult for the attackers. Rough terrain also allows greater opportunity for small unit ambushes and attacks. The Soviets can probably tell horror stories of what Afghans with decent anti-tank and anti-aircraft missiles in narrow mountain passes could do to convoys.
Logically and like Saddam in 2003, the Iranians wouldn’t attempt to disrupt the US buildup in the Persian Gulf to avoid giving the US an excuse to go to war as well as increase international pressure on the US not to invade. Presumably, like in 2003, this is ineffective and US troops launch an invasion of Iran. The goal would be decapitate the regime and install a friendly, let’s say nominally democratic regime in Iran, though any regime that collaborates with a foreign power to invade or occupy better hope that foreign power leaves significant troops to protect them from their citizens. Let alone “dead enders” from the old regime.
In addition, the Iranians have spent most of the last 15 years cultivating Shiite militias in Iraq as part of their axis of resistance. These militias dominate the Popular Mobilization Units (PMU) in Iraq, which largely can be expected to attack an American forces in Iraq in the event of war with Iran. Also problematic is the dominance of the Iraqi military and government by Shiites, many of whom have deep ties to Iran. They presumably would be less than willing to confront Iran on behalf of the US and while the US may be able to force Iraq to allow an invasion from that country to Iran, the reliability of the Iraqi military against Iran would be low. Any attempt to use Iraqi units against Shiite militias may well lead to the disintegration of those units and the collapse of the government in Baghdad. A coup by the PMUs with elements of the security and military could even occur in the event of a war.
The moment a US attack on Iran occurs, US troops in Iraq would be forced to fight full scale battles to secure their supply lines and rear areas even before crossing the border. The US would likely begin with airstrikes and missile launches against Iranian air defenses and air bases, military and government installations, and nuclear facilities. Given the overwhelming air and naval power available, these facilities are likely blown apart over the first few days with any unfortunate Iranian planes blown out of the sky or more likely destroyed in their hangers. Iranians retaliate with missiles targeting US ships in the Gulf, which may inflict limited damage on the US Navy, though I tend to think their effectiveness will be low. A couple of navy ships damaged by missiles or mines would be likely, though Iranians are likely to target Saudi, Kuwaiti, and Iraqi oil tankers and oil pipelines, where their missiles would likely be far more effective and damaging to the world economy. Likewise, they probably dump their stockpiles of mines into the Gulf to see if they get lucky. I would think that the Iranian navy has limited success against the massed might of three US aircraft carrier battle groups.
Fighting immediately erupts in Iraq as Shiite militias begin firing rockets at US bases and laying IEDs along US supply routes. They may try to seize control of Basra to disrupt US supplies, which would lead to heavy urban fighting in that city. Once the gloves are off, the Shiite militias would be armed with not only professionally manufactured IEDs but also relatively modern anti-tank missiles and rockets. US troops would no doubt clear the militias from Basra and along their supply lines to the invasion force, but its difficult and costly, and ties up troops that could be used in Iran as well as damage facilities needed to support the invasion.
While some US troops are fighting militias, the rest of the invasion force crosses the border. Most of the force attacks directly across the Iraq/Iran border, the first goal is to seize everything south and west of the Zagros mountains prior to a push through the mountains and the capture of Tehran. The city of Ahvaz is the target where various road and rail lines intersect, and will have to be taken if there is any hope of a major assault into the mountains. The road system largely dictates where the US will have to go in Iran. Unlike Iraq, Iran is not a nice flat desert.
Likewise, Ahvaz will be likely the first big stand of the Iranian military to see if they can stop the US forces short of the Zagros mountains. An addition stand would likely be conducted at Bandar-e Mahshahr, which is the closest port city to Iraq, and from there Iranian anti-ship missiles and rockets could complicate US attempts to use Basra. The most logical place to defend Iran would therefore be along the Ahvaz to Bandar-e Mashahr line about a 100 kilometers apart and about a 100 kilometers from the border, which gives a bit of space for stay behinds and guerillas to play havoc with the invader’s supply lines, gives the defenders a defense in depth away from the border, and two major urban areas to anchor the defense that will have to be stormed.
Both cities are likely encircled by US armored forces rapidly before a direct assault on both cities can be undertaken. Given how Hezbollah defended south Lebanon from Israel in 2006, odds are the Iranians would do the same. Rather than force big battles where US air power can slaughter the massed Iranians, let the invaders fight a hundred small engagements. Even if US air power intervenes in most of those engagements, at least some Iranian troops will be fighting on more even terms and sometimes they would be able to attack convoys and rear areas with limited combat effectiveness.
A second US force would likely need to land at Bandar Abbas, which is the base of the Iranian navy, and sits astride the narrowest portion of the Persian Gulf where Iranian anti-ship units and mine layers will need to be cleared away if oil exports are to resume. Let’s say one Marine division is tasked with taking the city, which with heavy air and naval support is likely possible if potentially painful as the defenders draw the Marines into house by house fighting for the port.
Let’s say two divisions to secure Basra and the supply lines from Iraq into Iran and another division for the invasion of Bandar, that leaves four divisions to cross into Iran and break the Ahvaz to Bandar-e Mashahr line.
After a couple weeks of fighting, it is nearly certain that US troops break the Ahvaz to Bandar-e Mashahr line and seize both cities after some bitter urban combat. Iranian stay behinds and guerillas will continue to attack supply lines and tie up troops but large formations of Iranian troops, likely including significant elements of the Revolutionary Guards, defending both cities have been destroyed. In the south, Bandar Abbas has been taken by the Marines after some bitter urban combat, but the Marines are fighting small unit engagements against Iranian units that are continuing to try to choke the Straits of Hormuz with mines and launch missile attacks against oil tankers. Without any further defenses short of the Zagros mountains, US troops push all the way there with light resistance but increasing the amount of terrain they need to control and lengthening their supply lines.
This is likely how the first phase of the invasion ends with US troops looking at the Zagros mountains and pausing to regroup and reinforce before trying to break through. Odds are that there are at least a couple thousand dead US troops and 20,000 to 30,000 wounded are evacuated out of the combat zone. Losses are heaviest among combat troops and actual combat effectiveness of the invading forces has likely been cut significantly to 2/3rds of their pre-invasion strength. Wear and tear on vehicles as well as combat damage and losses are probably significant as well. Additionally, constant attacks on supply lines and rear areas are causing continued losses and exhausting troops defending those areas. Probably 100,000 to 200,000 Iranians are captured, killed, or wounded in the first phase, but the bulk of the Iranian military is still intact for the next phase.
At this point, the US would likely have to rotate in additional combat troops, perhaps drawing in the 2nd Infantry from Korea and the 3rd Marines from Okinawa as well as a drawing up reserve units for the next phase of fighting. While US losses are not that heavy, they will fall among well trained and experienced combat troops, who will have to be replaced with relatively fresh recruits. It is quite possible that US forces wont be able to replace their losses without stripping rear and support units for more troops. The Iranians will be drafting every military age man they can put their hands on and throwing them into the fight. Their effectiveness will be limited but there is nothing like lots of bodies to use up.
Once the US breaks through the Zagros mountains, which will likely be far more costly than the initial fighting as many more Iranian troops are deployed in the mountains, there remains some rough terrain until Tehran is reached and the Iranians are likely to defend every hill and forest to the capital. Then the capital will have to be taken by storm with the remnants of the Iranian military and Revolutionary Guard making their last stand there.
There is a very significant possibility that the capture of Tehran does not actually end the war, as the Iranian high command can easily retain is authority over remaining Iranian forces and militias that can continue the fight. It would be impossible to secure all of Iran without significantly increasing the number of US combat troops in country and a draft may well become necessary to provide enough bodies to capture and hold every town in Iran. US troops could control any town or city that they are present in with any strength but outside their perimeter would be enemy territory. The odds of creating a friendly regime in Tehran that is actually able to leave the Tehran “Green Zone” without being immediately murdered by their fellow Iranians is likely to be poor, let alone control any portion of the country.
In short, the US won’t invade Iran because it is not an easy thing to do at all and risks blowing up Iraq as well. This is leaving aside Hezbollah’s immediate attack on Israel as part of the axis of resistance or potential uprisings in Bahrain and Saudi Arabia’s Shiite majority areas in support of Iran or inspired and directed by Iranian agents. If there is going to be a US-Iran conflict, it will likely be limited to some airstrikes and cruise missiles strikes with some retaliation from the Iranians, because the cost of all this is prohibitive to anyone not named Bolton apparently.