Donald Trump is stuck between “a rock and a hard place” after three weeks of war in Iran and “sending a message of weakness” to the world, Leon Panetta, a former US defence secretary and Central Intelligence Agency director, has told the Guardian.

Panetta, who served in the Bill Clinton and Barack Obama administrations, recalled that national security officials were always keenly aware of Iran’s ability to create an energy crisis by blocking the strait of Hormuz. That very scenario is now unfolding, leaving Trump with no exit strategy beyond wishful thinking.

“He tends to be naive about how things can happen,” Panetta, 87, who supervised the operation to find and kill Osama bin Laden, said by phone. “If he says it and keeps saying it there’s always a hope that what he says will come true. But that’s what kids do. It’s not what presidents do.”

Trump’s war began on 28 February with what it hoped would be a knockout blow. A surprise strike by Israel killed Iran’s supreme leader Ayatollah Ali Khamenei. The US and Israel soon gained air supremacy. But the longer the conflict has raged, the more that initiative appears to be slipping away.

Thirteen US service members and, according to Iranian health officials, more than 1,400 Iranians have been killed while Khamenei was succeeded by his son, Mojtaba Khamenei. Trump has struggled to sell the war at home as oil prices rise, his polling numbers fall and his electoral coalition shows signs of splintering. He has fumed at news coverage and sent mixed signals on objectives or when the “excursion”, as he terms it, will reach a conclusion.

Panetta said: “We replaced an old guy, a supreme leader who was near death at a time when the people of Iran were willing to take to the streets with the hope that they could ultimately change their way of government. And instead today we have a more entrenched regime, we have a younger supreme leader who’s going to be there a while, and he’s much more of a hardliner than the first supreme leader. That didn’t turn out too well.”

The regime has retaliated against the US and Israel by effectively closing the strait of Hormuz, throwing global energy markets into a tailspin. A fifth of the world’s traded oil flows through the waterway.

For Panetta, it is a crisis of the president’s own making. “This is not rocket science to understand that if you’re going to conduct a war with Iran, one of the great vulnerabilities is the strait of Hormuz, and [it] could create an immense oil crisis that could drive the price of fuel sky-high.

“In every national security council I’ve been a part of where we’ve talked about Iran, that subject always came up. For some reason, either they didn’t consider that could be a consequence or they thought the war would end quickly and they wouldn’t have to worry about that.”

He continued: “Whatever it was, they were not prepared for it and they’re now paying a price because, if there was an escape here for Trump, it would be to declare victory and it’s over and we’ve been able to be successful in all of our military targets. The problem is he can declare victory all he wants but, if he doesn’t get the ceasefire, he’s got nothing.

“And he’s not going to get a ceasefire as long as Iran is holding the gun of the strait of Hormuz against his head.”

Trump has said he does not plan to put US boots on the ground in Iran but is also sending thousands of marines to the Middle East in a possible sign of a coming operation. On Friday he declined to confirm a report by the Axios news outlet that he was considering an occupation or blockade of Iran’s Kharg Island to pressure Iran to reopen the strait.

Panetta said: “He’s facing a very tough issue, which is: does he go to expand the war by trying to get the strait of Hormuz open so that he can eliminate that leverage and maybe be able to ultimately negotiate with Iran? Or does he just simply walk away and declare victory, although everybody will clearly understand that he’s failed?

“It’s a very tough position he’s in right now but nobody else is responsible for where he’s at than Donald Trump.”

Help is not on the way. Last Saturday, Trump posted that other countries may need to help keep the strait of Hormuz open, the reaction was underwhelming. On Friday, Trump branded Nato a “paper tiger” without the US and mocked its members as “cowards”. He kept allies other than Israel in the dark about his war plans for Iran.

Panetta commented: “If you’re planning a war, it’s not a bad idea to talk to your allies. Alliances are important to be able to support any kind of military effort. We’ve learned that lesson going back a long way to world war two. But he [Trump] takes a callous approach to alliances and now he suddenly finds himself in a place where he’s got to turn to allies, to Nato and to others, all of whom he certainly hasn’t treated well in his presidency, to try to help bail him out.”

The former defence secretary added with a chuckle: “The chickens are coming home to roost.”

He advises Trump to abandon his magical thinking and “face the fact” that he must use the military to open the strait, neutralise Iranian defences along the coast and deploy ships to escort oil tankers through.

“There’s no question there’s going to be lives lost and it’s clearly going to expand the war but I don’t see the alternative. He’s got to do it. He’s talked a great deal about the strength of the United States. This is a test of whether the United States can be able to deal with that situation which otherwise is not only going to prolong the war but create a lot of economic damage to the United States with those soaring fuel prices and cause what some have said is a potential worldwide recession.”

Panetta added frankly: “There’s not much choice. You’ve got to do what you have to do and, if you can open the strait, it might give you a better chance to then have a basis on which you can negotiate hopefully some kind of ceasefire. That’s the only way that he can go at this point; otherwise he will clearly have failed to find a solution.”

An ex-army intelligence officer, Panetta was White House chief of staff in the Clinton administration then served as CIA director and the 23rd secretary of defence under Obama. He is now chairman of the Panetta Institute for Public Policy based at California State University, Monterey Bay. His son, Jimmy Panetta, is a Democratic member of Congress from California and former navy reserve intelligence officer.

He is not impressed by the bombastic antics of Pete Hegseth, a former Fox News host who now occupies Panetta’s old office at the Pentagon. “He’s not a secretary of defence. He is simply an enabler for whatever Trump wants him to do.”

Panetta also condemns a recent run of meme-style videos released by the White House that juxtapose war footage with Hollywood films, video games and sporting action, as well as a fundraising email that used a photo of Trump at a dignified transfer of remains of soldiers killed in Kuwait.

Panetta said: “When he or those around him started publishing pictures of football games, raising money by using pictures of our dead coming home at Dover [air force base], and doing the kind of tasteless things that he can do, he’s basically sending a message of weakness, not a message of strength to the world.

“That, unfortunately, is what the world sees right now, and I can see why he’s having problems trying to get allies to be able to respond when they’re not sure he knows what he’s doing.”

Born during the presidency of Franklin Roosevelt, Panetta has never seen a commander-in-chief shatter norms as Trump does. When a Tomahawk missile hit a girls’ school in southern Iran on the first day of the conflict, killing at least 175 people, most of them children, Trump sought to blame the attack on Iran, claiming its security forces are “very inaccurate” with munitions.

“Any other president of the United States would have recognised the mistake and apologise for what happened,” Panetta remarked. “He doesn’t do that. It sends an image of America that kind of fits the ugly American image that a lot of people once had of this country.”