Broadly speaking, the US is okay with invading Iran because Iran isn't a US tributary and the Iranian state is unwilling to become a tributary because theocracy is incompatible with US tributary status (A king might swear fealty to an emperor, but god's emissary can hardly submit to a heathen).
The primary objective of the US administration is to win the midterm elections. War has historically boosted Republican support under low-informations voters and centrists, and going by the standing ovation in Congress, Democrats are happy to go along with the narrative that this is a just war because they too support enforcing tributary status.
That takes care of the motive. The means is the US military-industrial complex. The opportunity comes in the form of the US breaking a nuclear non-proliferation treaty with Iran and then being outraged that Iran isn't complying.
They could count on Iran not complying because Israel keeps attacking Iran (and its other neighbors), ensuring Iran will defend itself in a way that can be treated as offensive. Israel keeps attacking its neighbors because that is what the US pays them for and because their history of doing that means that if the US cut funding for unrendered services, many Israelis would have to flee to escape facing justice for their participation in genocide.
So that's means, motive, and opportunity for the US attacking Iran. Now how will it go?
Firstly, the motive being elections means the US will keep the war going until at least after the midterms. Economic consequences of this can be spun as justifying Republican autarky. The biggest consequence would be oil shortages in most of the world. Countries with large domestic production and/or strategic reserves, including the US, Canada, China, Russia, and Venezuela, would be less affected or have their position strengthened (with Venezuela going along with the US because they now know what happens if they don't).
In Iran, the US will keep bombing targets until it is satisfied ground resistance is sufficiently mollified, then move troops in to occupy. Given Iran's geography and the theocratic nature of the current regime, it will likely be a phase of shattering state power followed by a phase of occupation and guerilla warfare.
The US will attempt to create safe zones from which the tributary government and military loyal to the US operate and grow in power, likely centered around urban areas, key infrastructure, and strategic resources. Given there is more support for regime change in Iran than in Afghanistan, the tributary government might attract enough loyal troops to slowly take over the fight against the loyalist guerillas and not collapse immediately when US support drops.
This invasion has bipartisan support so even the Trump administration being replaced would not stop it. I don't know how long the Iranian state can resist, but as long as it does the blockade on the Strait of Hormuz will remain in place, and even after that guerillas will make that route unsafe. After that, getting the tributary government shipshape could take years or decades, on the long end probably getting cut off like Afghanistan.
Iran or its supporters may attempt asymmetric warfare. If the US government wants to, it could replicate 9/11 and its massive boost for Republican popularity by having a similar lack of curiosity about suspected upcoming attacks. There have been articles about a possible Iranian drone attack from a ship off the coast of California which could fail to be stopped. Similar attacks may occur on the rest of NATO, and slowly ebb as Iranian loyalist power diminishes.
The economic consequences of an oil shortage would naturally hit vulnerable targets hardest. Food delivery vehicles might not be able to afford a trip into remote rural areas during a famine, while Europe can rely on electrified modes of transport for day-to-day stuff and reserve oil for essential services.
Countries with more oil, such as Russia and the US, may take advantage of this situation. Ukraine is not looking so good.
At any time, the US could move on to the next crisis and either stretch itself thin or leave the Iranian tributary state without support in what would then be a civil war. Israel will continue its aggression until its funding gets cut and it collapses.
So it goes.
