I think the problem with the Iran-Iraq war was that after they exhausted their imported late 20th-century tech, they reverted to the level of war the sides could support - equivalent of WWI trench warfare… complete with human wave assaults, artillery duels, and lack of movement of the fronts.
The grossest stupidity of the Iraq invasion in 2003 (apart from the invasion itself) was firing the infrastructure of the nation. Even after WWII the allies rcognized that to efffectively govern and keep a lid on things, they needed the policing infrastructure of the Nazi regime until they could sort out who to fire. Iraq was left with no control, and the now unpaid experts on things military were motivated to get even. Add to that, the Shiite-Sunni divide and teh realization by Sunnis that they were going to be targetted by a bigger Shiite population, and they were additionally motivated to stockpile the means to carry on the fight. I also read somewhere that the troops were also planning from the start the fade-and-harass tactics that caused the most trouble. Al Qeda in Iraq was just a bonus…
Then the USA compounded the error by banning Baath party members from many positions. More motivation; more people to jump in. Anyone with education - law, engineering, teaching, etc. - generally needed to join the party to get a government job much as the Communists worked. It was a mistake to equate all these people with Saddam loyalists; it was stupid to turn them against the occupiers.
The other mistake is to assume invaders would be welcome with open arms just because the government was a dictator. If China invaded to save the USA from the Republicans/Democrats, do you think the other party’s supporters would join with the invaders, or would they join together to defend their homeland? Beare about pollyannistic assumptions of other people’s motivations.
The Iranians, for all the appearances of fanaticism, are not stupid either. I’m sure they’ve studied the tactics involved in Iraq, and come away with the best options - lots of hidden weapon caches, IED supplies and studies where to use them, and such. Plus, they haven’t been as starved as Iraq for supplies or equipment. Their best tactic would be to “surrender”. Come on in, please don’t bomb our bridges and power plants, don’t destroy our infrastructure. Surrender the most useless half of the army and half the government that would be captured anyway. Once the occupiers are in place, then the insurgency begins. Why waste men and material in the fight you can’t hope to win, when you can save them for the fight you can win?
The USA would be better off sending in the troops, taking the key positions like nuclear plants and processessing facilities, anti-aircraft installations, air force fields and planes, etc. with commando raids, blowing them to useless, and then walking away within a month leaving the smoking ruins. Leave as few of the old guard government behind as they can manage.
The USA would be best off with no military action.