As US and Israeli jets descended to deliver the opening salvos of the war in Iran, Donald Trump’s back-of-the-envelope plan for regime change in Tehran was about to run into the reality of the largest US intervention in the Middle East since the start of the Iraq war in 2003.

That reality came quickly.

One hundred and seventy-five people were killed when a US Tomahawk missile slammed into a girls’ school, apparently because the Pentagon used outdated targeting data for the strike. Hundreds of air-defence missiles were expended as Iran’s initial missile counterattack was mostly parried – but one drone smashed into a makeshift command centre in Kuwait, killing six US troops and wounding dozens more.

Tens of thousands of US citizens were stranded in the region as the state department hurriedly slapped together a taskforce to evacuate them. The US strikes that killed Ayatollah Ali Khamenei also killed many of the US’s preferred successors; and in his first address, Trump simply told Iranians “when we are finished, take over your government” – with no suggestion on how that might be done.

And the first six days of the war alone cost the US $11.3bn, the Pentagon briefed members of Congress – though it wasn’t clear if those numbers included the cost of the buildup or the US missile defenses as well. The ultimate toll of Iran’s closure of the strait of Hormuz on the world economy remains to be seen.

Past administrations had been war-gaming an Iran invasion for decades – but with Trump in the White House, observers said that the rigidly closed circle of advisers around him, the collapse of an interagency process in the government and his erratic decision-making process made this unlike any other US military campaign in recent memory.

“This is hard under any circumstances but especially with so little [evidence of] planning,” said Philip Gordon, the former national security adviser to Kamala Harris and the White House coordinator for the Middle East under Barack Obama.

Of the growing chaos in the Middle East, he said: “It is surprising that Trump is surprised.”

Previous administration had “gamed out” potential conflict scenarios with Iran “many times and constantly”, said Gordon, now of the Brookings Institution, but regularly ran into exactly the problems that the Trump administration is now facing: Iran targeted neighbouring countries to threaten a regional war and closed the strait of Hormuz, threatening the global oil trade and driving up energy prices.

“One of the reasons we did the nuclear deal and didn’t try to change the regime is exactly what’s happening,” he said of the 2015 Joint Comprehensive Plan of Action (JCPOA). Trump withdrew from the treaty in 2018.

The military campaign to eliminate the Iranian leadership has achieved considerable success. The early strikes that killed Khamenei and dozens of his senior advisers were the product of a collaboration between Israeli on-the-ground intelligence and US signals intelligence. Trump seemed poised to match the success of the 12-day war, when the US delivered surgical strikes against Iran’s nuclear programme and then exited the conflict.

But Iran has continued to fight. And as Trump and top officials such as the US defence secretary, Pete Hegseth, herald the complete destruction of the Iranian leadership in successive briefings, there are no clear explanations for what the US will call victory in the conflict, and how it will now reverse Iran’s decision to squeeze the global oil supply.

“The military planning has been stellar,” said Michael Rubin, a senior fellow at the American Enterprise Institute and an expert on US foreign policy on Iran. “That said, politically, this is increasingly looking like a cluster fuck. And the reason is that step one of any plan is to establish a goal – the targeting should be in pursuit of that goal. The United States has this backwards. We have the targeting, but we don’t have a clear goal, and that lies not on the Pentagon planners, but on Donald Trump.”

The aim of the US mission has shifted repeatedly since January’s naval buildup – from backing Iranian protesters being killed in a government crackdown, to eliminating Iran’s nuclear programme, to knocking out its ballistic missiles system. Now it has focused on a new goal – that of opening the strait of Hormuz, whose closure has sent prices for oil to more than $100 a barrel and even prompted the Trump administration to halt sanctions on Russian oil, reversing its policy in a different war.

“Each of these goals would have required a different military strategy,” said Michael Singh, managing director of the Washington Institute and a former senior director for Middle East affairs at the national security council under George W Bush. With Iran now closing the strait, he added, the “other side gets a vote” about when to end the war, potentially allowing Iran to drag the US into a protracted conflict.

The small-circle decision-making was in part by design.

Trump entered government last year with a broad attack on the “deep state” that others have called the DC foreign policy “blob”, cutting out the career government employees that had populated agencies and departments and that he argued acted in conspiracy to undermine his previous administration and prevent changes to US foreign policy. Within months of his inauguration, Trump gutted the national security council, and later the secretary of state, Marco Rubio, proceeded with major layoffs called at the Department of State.

There were few signs that key parts of the state department beyond Rubio’s immediate circle were brought into the policy planning: there were no operations to evacuate citizens and at-risk embassies remained staffed even through the beginning days of the war. But even as the US’s intention to go to war had been telegraphed, the closely held decision to launch the strikes meant that official directions had not been handed down to the state department and other key agencies.

“This is the war we launched,” said Mara Karlin, a former assistant secretary of defence. “Obviously I understand the need for operational security, but you have also got to set certain pieces in place so that you can be ready to respond when things happen.”

The Trump administration has also not voiced a clear plan for the people of Iran. Trump himself has indicated he wanted someone inside Iran to seize power – similar to the case in Venezuela – but then said many potential figures from within the regime were killed in the initial strikes. He has now conceded that a change of regime likely won’t happen in the short term.

“They literally have people in the streets with machine guns, machine-gunning people down if they want to protest,” Trump said of Iran’s security forces. “That’s a pretty big hurdle to climb for people that don’t have weapons.”

For Pentagon planners, the expanding war has meant drawing resources from other military theatres, including parts of air-defence systems deployed to Asia in order to contend with the long-term threat from North Korea and China. Flush from its success in Venezuela, the Trump administration has now embraced the use of military power abroad to achieve its aims, potentially overextending itself in a conflict that could draw in much of the region.

“The long-term effects of this will be just dumping US military power down the drain,” said Jennifer Kavanagh, director of military analysis for Defense Priorities, a thinktank based in Washington DC that generally advocates for more restraint in US foreign policy.

“The long-term effects will be significant in terms of the US ability to project power … I think that the implications of this are going to last for decades.”