1) one can't compare Iraq of 1991/2003 with Iran----because Iraqi military in 1991 was designed for POSITIONAL WARFARE and its command and control and supplies were extremely vulnerable to US air power. Once US established air superiority, it used its air power to easily paralyze Iraqi army's supply and command and control before US ground force attacked.
If Iraqi military was designed in a different manner, war in 1991 could have been different. (One example is how ISIS tactics allowed them to survive US air strikes).
War with Iran will look like war between Israel and Hezbollah in 2006.
Hezbollah was prepared for a war with a country that enjoys full air superiority. As a result Israeli air strikes and artillery were extremely ineffective and when Israeli ground forces invaded Lebanon, they were stopped by anti-tank missiles and Hezbollah infantry force.
2) USA can't invade Iran because of its size and terrain.
In 2003 to invade Arabic part of Iraq that had a population of 20mln people, US required 250.000 troops and Donald Rumsfeld claimed it is not enough, (even despite intelligence estimated showed that there will be no resistance from Iraqis).
Iran is 4 times bigger in terms of population and several times bigger in terms of territory.
If 250k troops are required for invading a 20mln country, then to invade 80mln Iran you will need 1mln troops. More realistic estimates claim 1,5-2mln troops are needed to occupy such a big country like Iran. These resources are beyond US capabilities.
Also, USA military spending on Iraqi occupation was 150bln$ a year,(even despite virtually no resistance against American forces).
Based on this number, we can estimate that to invade Iran --military spending will be 800bln$ and more realistically 1-1,5 trln a year.
US WILL GO BANKRUPT! US simply has no money and resources to occupy such a big country like Iran (for this they will have to mobilize huge portion of their population and economy and go to WWII style total war).
Also, US military is designed for war in Europe and is effective in a flat terrain of Iraq. But Iran is a mountainous country. Armored brigades and divisions are ineffective in a mountainous terrain. Mountains are hell for invader and a paradise for defender.
It will be very difficult for US forces to penetrate the mountains and supply its forces, while Iran will easily defend its territory and ambush supply convoys. Every single hill will be turned into a fortress.
Mountain war is an infantry war and this is where Iran has advantage, because its military is specially designed for this kind of warfare --war in a mountainous terrain against an enemy that has technological superiority.
Conclusion is that US can't occupy Iran, but it can make a limited invasion occupying Khuzestan province and islands in the Persian Gulf, (and later trade to return them for peace). But for invading Khuzestan US will need to create a spearhead and amassing troops takes a lot of time.
3) US calculates that if it will attack Iran, 200.000 troops Iran has near the border with Iraq will attack American forces in Kuwait and oil infrastructure of Saudi Arabia.
So for protecting Kuwait, US will have to bring its own ground force to the region and the deployment process will take 5-6 month---a LONG PERIOD OF TIME.
Probably Iran will not repeat Saddam's mistake and wait for 6 month watching how Americans deploy forces. Maybe Iran will use its large ground force to strike first.
Also 10 years ago there were rumors that if US starts bringing large forces to the region, Iran planned to sink its own oil tanker in the Strait of Hormuz to block it and paralyze US deployment.
4) To attack Iran, US will rely on its air power which was extremely effective against Iraq in 1991 and will be ineffective against Iran today (just like against Hezbollah in 2006).
US will bring 1600 fighter aircrafts and its support infrastructure to the 30 airfields of Saudi Arabia and these aircrafts will be parked like this
https://en.wikipedia.org/wiki/Gulf_...15E_parked_during_Operation_Desert_Shield.jpg
These aircrafts will become an ideal target for hundreds of Iranian ballistic missiles with cluster munitions.
5) Even is US manages to deploy its forces, the main phase of war will last for 2-3 days.
During the first 3 days, a) Iran will launch all of its ballistic missiles and damage US air force and oil infrastructure/water supply infrastructure in Saudi Arabia/Kuwait/UAE b) Iranian navy will lay mines in the Strait of Hormuz c) Iranian anti-ship missiles will destroy oil tankers in the Gulf, possibly oil tankers in the Strait of Hormuz.
By the 10th day of war---Iranian navy and air force will be destroyed, but it will be
TOO LATE----Strait of Hormuz will be blocked, oil tankers will be sunk, ports and oil infrastructure in the Arabian Peninsula will be damaged/destroyed.
It will take 1-3 days to destroy oil infrastructure of the region and it will take many months to restore it and resume oil traffic.
6) Shia rebellion in Saudi Arabia will do even more damage to oil infrastructure of the region and create permanent instability that will keep oil price high.
pro-Iranian militia in Iraq can paralyze oil supplies from Iraq and Kuwaiti oil infrastructure is vulnerable not only to Iranian missiles but also to Iranian ground force.
And even if Strait of Hormuz is reopened---a single anti-ship missile launched at oil supertanker can block the Strait one more time.
Persian Gulf supplies 25% of world's oil and 50% of oil exported by sea.
If the Strait of Hormuz will be blocked, oil price will skyrocket---this will generate inflation and physical deficit of oil in the world---stock markets will crash, banks and investors will lose their money and will go bankrupt, business will default on debt generating a debt crises and recession.
And who will be blamed for a massive economic crises?--US president! Congress will blame the President for starting illegal war of aggression without authorization of Congress---a war that made serious damage to US economy and welfare of American people----a war that damaged interests of United States----President will be impeached.....
In short:
1) US air force can attack Iran---but US can't invade Iran
2) in the first day of war Iran will lay mines in the Strait of Hormuz, launch ballistic missile at oil terminals in the Arabian peninsula and anti-ship missiles at oil tankers in the Persian Gulf
3) By the 10th day of war Iranian Navy and Air Force will be destroyed but it will be TOO LATE for USA---oil infrastructure will be destroyed and oil supply will stall.
4) it will take months to reopen the Strait of Hormuz and rebuild oil infrastructure of the region and until then global economy will collapse---US president will be impeached for the damage he has done
5) Instability in Shia regions of Saudi Arabia will become permanent keeping oil supply at risk and oil price high