Tehran’s Grand Mosalla Mosque, where the late Ayatollah Ali Khamenei was held in state this weekend, is an extraordinary building. In a country filled with exquisitely tiled, millennium-old houses of worship, this one is a plain, brutalist product of the 1979 revolution whose sole virtue is its size. With a main prayer hall that spans multiple World Cup football fields, it is a still unfinished monument to the sheer scale of religious fervour that Iran’s supreme leader sought to institutionalise.

When I went to Friday prayers in the mosque a decade ago, it was clear Khamenei had failed. Attendance was sparse. When the cleric leading the service began punching the air to chant “Death to America! Death to Israel! Death to the Infidel!” the response was mechanical and insincere. As an American infidel, I didn’t take part. My neighbour asked where I was from and brightened visibly when I told him. “Ah, I like America,” he said, citing its freedoms.

This week’s ceremonies are unfolding in the same place but a different world. There are far too many mourners for the indoor prayer hall and the sense is of defiance and fury. The “Death to America!” chants are vengeful and heartfelt. When I visited, the 2015 nuclear deal was still in effect and many Iranians had hoped the days of their nation’s zero-sum confrontation with Washington were over. Now the countries have been at war and the regime is trying to revive the legitimising myth that lost its power in Khamenei’s later years and left him ruling by violence alone. The war has made a cruel, despised and out-of-touch old man into a martyr.

Khamenei’s six-day funeral is high political theatre. The state invited world leaders and Iranians from around the country to come to Mosalla and show their love and respect. The procession is supposed to move to other religious centres including Karbala, in Iraq, where just a week ago Shias converged from all around the world to celebrate their faith’s origin story in the martyrdom of the Prophet Muhammad’s grandson, Hussein ibn Ali. The timing, more than four months after Khamenei’s killing, is choreographed. The connection is intended to be made.

Pro-Iran militias in Iraq called on their members to turn out in force. Harakat al-Nujaba leader Akram al-Kaabi told them in a Telegram post that their presence would be “no less significant than fighting these vile Zionists on the battlefields of Jihad”.

In all, Iranian officials say they expect 20 million people to pay their respects to Khamenei, which would make this the largest funeral in recorded history. Nobody will be able to count the true number who attend this rolling ceremony, but it’s likely to reach into the millions, and why not? Josef Stalin also built a state on cruelty and repression, killing vastly more of his own people than has Khamenei. Millions turned out to mourn his loss, too, and he wasn’t killed by a foreign power, alongside his 14-month-old granddaughter. Stalin died of a stroke after a drinking binge.

Iran’s leaders, including Khamenei, were out of ideas back in January. The vast majority of the population loathed them. The revolution and subsequent eight-year war with Iraq that shaped their world view meant nothing to this year’s protesters, most of whom hadn’t been born in 1979. The Islamic Republic needed a new legitimising myth. It hopes it has found one now in the story of Khamenei’s martyrdom and the Islamic Revolutionary Guard Corps’ defence of Iran against the combined military might of the US and Israel.

For IRGC commander Ahmad Vahidi and other regime leaders, this funeral is probably more important than their 60-day negotiations with the US. It is impossible to know whether this attempt to rejuvenate the Islamic Republic will succeed. Even if 20 million people do show up to look at Khamenei’s coffin, in a nation of more than 90 million that would merely confirm what we already knew — that the regime still has a bedrock of faithful supporters, even as it lost the majority.

And yes, a handful of international leaders did show up for the funeral, as did a few lower-ranking officials from the likes of China, Russia, India and — most significantly — Saudi Arabia. But this showed less Iran’s strength than its continued international isolation.

The economy will remain in terrible shape when this funeral is over. Year-on-year inflation will still be closing on 90 per cent. The replacements for Khamenei and other leaders killed in the opening hours of the ridiculously named “Operation Epic Fury” will face the same difficult decisions as before. How far can the Islamic Republic afford to relax rules on female dress, rock concerts, dissidence and other hallmarks of the decadent West it’s fighting, before it stops being the Islamic Republic? How does the complex coalition of regime factions that now rules the country restore the economy without reconciling with the US or loosening the IRGC’s corrupt grip on much of it? How does the regime secure the nation’s water supplies, after failing to husband them for decades?

These questions and more will return once the war’s rally-around-the-flag effects fade. Which is why Vahidi, arguably now the most powerful man in Iran, may not be in a hurry to see the conflict end, unless that happens on terms that look like a clear victory and leave the Islamic Republic with more money and options than it had before the war.

Many will say this argues for a return to war to “finish the job”. But if Operation Epic Fury and Khamenei’s funeral drama prove anything, it is that airstrikes can’t achieve the regime change that such finality would require. They just undermine the story that the West has to offer like-minded Iranians, and make the job harder for those who would actually have to finish it. For if and when that happens, American and Israeli fighter pilots won’t be the ones responsible. That will fall to Iranians, drawn from the 70 million-plus people who didn’t take part in the IRGC’s funeral theatre.

• Marc Champion is a Bloomberg Opinion columnist covering Europe, Russia and the Middle East