Iraq is a fantastic lesson to heed today.
In the first gulf war, Bush Sr. refused to occupy the country. He viewed it as too difficult and too expensive. In the second gulf war, Bush Jr. declared victory from the deck of an aircraft carrier, occupied the country, hunted and executed its leader, and then opened the U.S. treasury to deal with the aftermath. Thousands of Americans died. Hundreds of thousands of Iraqi's died. The occupation was long and difficult, but its end was still premature and left a power vacuum that ISIS raged into, causing even more destruction. Perhaps Iraqi's can say they're better off today than under Hussein, but a terrible cost was paid. Most of the blood was Iraqi, but most of the treasure was American.
The financial drain on the U.S. was extreme enough to expose the world's preeminent superpower as being unable to bring the occupation of a somewhat backwards and minor dictatorship to a successful conclusion. Iraq is not a big country, in either population or area, but it was still too much for the U.S. to control, even with willing allies. This failure made the world realize there were severe limits to what the U.S. can do. Sure, it might defeat the military of a middle or even major power, but occupy and control it? Fat chance!
In the days ahead, the U.S. military is going to bomb anything that moves and looks like it might shoot back, as well as a lot of infrastructure and probably a decent number of civilian targets by mistake (or design). Trump has framed this invasion as being directed towards eliminating Iran's nuclear program, so expect a lot of facilities in close proximity to civilians (and many of those civilians) to be vaporized.
If Trump is listening to his generals even slightly, he will not try to occupy the country. He'll declare victory and move on to whatever outrage is next to maintain his "Flood the zone" strategy and keep the Epstein heat from finally catching up with him. If that's all he does, this will be another war like Bush Sr.'s. Expensive, but not ruinously so. U.S. deaths will be in the hundreds and not the thousands. Iran will most likely fall into the hands of another mullah or descend into chaos, becoming a long-term security quagmire that will probably continue to bleed the U.S. for decades to come. Even if democracy does take root in Iran, it likely won't be a democracy that's friendly to the U.S..
If Trump isn't listening to his generals (who reportedly advised against the invasion to begin with), he might try to occupy Iran. Iran has double the population and four times the land area as Iraq. Unlike Bush Jr., Trump has not even tried to stitch together a coalition to share the costs. It's unlikely that many countries would be dumb enough to sign on now. There's no NATO article 5 pretext to drag in other NATO countries. There isn't even a falsified pretext like WMD's to quiet the howling in the UN. Israel isn't the kind of help the U.S. needs because the U.S. pays most of Israel's military bills to begin with. In short, if Iraq strained the U.S.'s finances close to the breaking point, Iran will ruin them completely. There's absolutely no way the U.S. can afford to occupy Iran.
Even if Trump cuts and runs, this war will ensure American's can't afford socialized medicine for another generation.