The world’s busiest airport is shuttered. Hundreds of drones are swarming across several borders. Millions are sheltering in place. Millions of others are in mourning.

Rarely, in the Middle East’s recent history, has a violent episode in one country spilled over to so many of the region’s corners, and in such short order.

But in the span of the first 48 hours, the sweeping U.S.-Israeli attack deep inside Iran and the subsequent killing of its supreme leader immediately unleashed an Iranian response that has now touched, in one form or another, 10 Middle East countries: damaging infrastructure, claiming lives, opening at least one new front — and foreshadowing a profound shift in the political ground across the region.

And though U.S. President Donald Trump said Sunday that Iran’s military losses have left it "very substantially weakened," he promised the deadly aerial onslaught will continue, possibly for weeks.

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Iran, too, is promising more counterattacks, apparently aimed at widening the cascade of consequences, are yet still to come.

What's next for Iran after the supreme leader's killing?

But even with a longstanding backup plan for the possible elimination of its supreme leader, and even if its institutions ultimately do survive the pressure — from outside and from within — Iran’s authoritarian regime is in its most precarious state since the revolution that catapulted it to power almost a half century ago.

And whenever this conflict is over, even in the absence of complete regime change, Iran will likely, indeed, emerge diminished, and that would be the region’s most transformative change in generations.

Exactly how this new state of affairs unfolds will depend, of course, on what remains of the existing Iranian posture — toward its neighbours, Israel and the West — its political stability and its treatment of its own citizens. A new phase of political repression, for example, or a civil conflict could send thousands of refugees across borders and invite outside intervention. A further economic collapse could bring instability of another kind. 

Politically, in the short term, Gulf Arab states that had been on a path of rapprochement with Iran — or those that had favoured mediation efforts — are now, after being attacked, will be forced to reassess their approach if any part of the regime survives.

“Regardless of how the broader conflict unfolds, the impact on Iran’s regional credibility is already significant,” Khalid Al-Jaber, executive director of the Doha-based Middle East Council on Global Affairs, told the Atlantic Council, a U.S. think-tank.

By attacking countries that had been pushing for de-escalation, he said, Iran "risks deeper isolation and stronger regional alignment against it."

If the conflict they sought to avoid deepens, a decision by Gulf countries to become involved could also have lasting consequences.

In the bigger picture, the region’s longstanding Sunni-Shia divide may still diminish as a driving force and as a result, redraw the region’s political map.

Meanwhile, the weakening of a centrifugal force of anti-American and anti-Israeli sentiment could also potentially galvanize efforts to further normalize relations between Israel and the rest of the region.

Paramount for Israel, a less hostile Iran means the neutralization of its biggest regional foe — and the threat of it becoming a nuclear power — that Prime Minister Benjamin Netanyahu has been warning against for more than a decade.

If a less stridently anti-Israel leadership takes over, that could dilute or even put an end to the so-called "axis of resistance" of countries and groups that opposed Israeli and U.S. influence in the region and beyond, with Iran at the helm.

Elements of those groups could continue to operate and coalesce and form new alliances. But with the ousting of Syria’s Bashar al-Assad, and the significant degrading of Hezbollah in Lebanon and Hamas in Gaza by Israeli forces following Hamas’s October 2023 attacks — the reach of that axis was already diminished before the current conflict. 

A weakened Iran would weaken that axis further. 

For a country like Iraq, a weakened Iran could also diminish the influence of the anti-U.S., Iran-sponsored militias operating within its borders. 

And yet again, Ayatollah Ali Khameini’s killing, mourned by millions of Shia Muslims across the region, may lead to new unrest that unsettles any notions of regional peace even if Iran is weakened. Hezbollah's attacks and Israel’s counterattacks in Lebanon early Monday are a case in point.

There are yet other factors that could influence a diminished Iran’s behaviour — and the region’s future: what becomes of Iranian oil, currently mostly bought by China? And how do China and Russia react to a transformed Iran?

A changed Iran is now inevitable. The supreme leader’s killing alone, after 36 years in power, will invariably begin to shift the country’s posture, Arash Azizi, Yale University historian and author most recently of What Iranians Want: Women, Life, Freedom, wrote in The Atlantic.

"Even in the not-quite-best-case scenario, Khamenei’s demise will likely allow Iran to abandon some of his most destructive core policies in the short-to-medium term — not least, his insistence on sacrificing Iran on the altar of a failed ideology," he wrote.

Could U.S.-Israeli strikes on Iran turn into ‘Iraq 2.0’?

Azizi said a more pragmatic remnant of the current regime that is conciliatory toward the U.S. might be willing to negotiate, and perhaps even establish diplomatic ties, with Washington, maybe even relax rules around women’s hijab. 

But all that would still "fall far short of what so many Iranians, myself included, have fought for—what thousands have only recently died for," he wrote.

The Middle East may have to wait months, maybe years to find out the kind of Iran they inherit after this conflict is over.

The fate of its beleaguered people, thousands of whom were recently killed for demanding basic rights, hangs in the balance.