Here is what I think happened on the Iran misadventure:
1) the proponents of regime change hoped and prayed that murdering the Ayatollah and IRGC leadership would result in a domestic uprising. But they’re pretty smart - and knew this was at best 50/50;
2) they thought that once the USA was knee deep in this business - they could drive Trump into a full blown war to effect ‘regime change.’ Boots on the ground. And they thought the American population would puppy-dog after them. This was deeply cynical - they thought they could manipulate public opinion into a war of choice, slow-walk them into something that no one wanted;
3) the American people are totally fatigued with Middle East nonsense - they want nothing to do with sandbox feuds over there. The war proponents underestimated the extent of opposition - and really don’t even care what people think, because they are monomaniacal about this;
4) no effort to sell this ‘war’ was even made. That actually shows that someone in the administration was using his brain, because it wasn’t a sale you could close. But that also meant that you couldn’t maintain this for long. The Bush administration sold us the Iraq war for the better part of a year - with lies and misinformation - but they at least thought public opinion mattered. The Trump administration made no effort, probably because they knew it was impossible;
5) for whatever reason Trump decided to proceed with a war that could only succeed if it was over in a couple weeks. It was a mistake, as was withdrawing from the Obama treaty - even if that wasn’t a good treaty. Because if you cannot gain American public opinion for a war, you cannot sustain it for long. And that means you cannot win the war if your enemy is prepared to hunker down and take punishment and wait you out;
6) I think all of this was predictable. You cannot engage in colonial style warfare, as though the USA were the sole superpower, if you cannot generate domestic support. That is the point people in the Trump administration should have known - and almost certainly did know, but rolled the dice anyway;
7) the upshot IMO is that the USA should only project power in its own hemisphere, and even that modestly. It should seek to reform domestically to be a stable republic - and not try so hard to force everyone into its mould. There is a lot to like about the USA - and making this place better, and showing it to the world, is better diplomacy than killing people in sneak attacks.