The fall of Syrian president Bashar al-Assad has dealt yet another major blow to Iran's so-called "Axis of Resistance", experts say.
For decades, Iran has been building up its network of like-minded militant groups and proxies to counter US and Israeli power across the Middle East.
But the alliance has been significantly weakened over the past year.
Now, with Iran's longtime ally Assad gone, and rebel groups that oppose Tehran in control in Damascus, the future of the resistance is being questioned.
Why the Axis needed Syria
Besides Iran, Syria was the only state actor in the Axis of Resistance, alongside proxy groups such as Hamas in Gaza, Lebanon's Hezbollah, and the Houthi rebels in Yemen.
Syria played a crucial role in the alliance, often described as the "key chain" in the movement.
It was the important geographical link that allowed Iran to move weapons and other supplies to Hezbollah in Lebanon.
[map]Assad, who assumed power in 2000, had acted mainly as a curator, Ali Mamouri, a research fellow in Middle East studies at Deakin University, said.
He allowed several militant groups, including Iraqi militias, Hezbollah and other members of Iran's proxy network, to operate freely across much of the country and move weapons, finance and aid throughout the region.
Dr Mamouri said without the Assad regime, support to Iran's key ally, Hezbollah, would be largely cut.
"Hezbollah will be more isolated in Lebanon," he told the ABC.
"It's already under social pressure in the country, it's under Israeli military pressure, and this will add more to the crisis for them."
Proxies already weakened
Danny Citrinowicz, a non-resident fellow with the Atlantic Council's Middle East Programs, said the fall of Assad was "another nail in the coffin of Iran's Axis of Resistance".
It follows significant damage Israel has already inflicted on Hamas and Hezbollah.
Hamas has faced more than a year of Israel's crushing campaign in Gaza, triggered by the group's attack on the country on October 7 last year.
The attack on Israel left about 1,200 dead, and another 250 were taken hostage to Gaza, according to Israeli authorities.
Israel's subsequent strikes on Gaza have claimed the lives of more than 43,000 Palestinians, according to the Gaza Health Ministry.
In July, Hamas political leader Ismail Haniyeh was assassinated while in his residence in Iran.
He was replaced by Yahya Sinwar, who was also later killed in an Israeli strike in the Gaza Strip.
Sinwar was described as one of the masterminds of the October 7 attacks.
The war in Gaza spawned another in Lebanon, with Israel conducting heavy air strikes and ground assaults against Hezbollah.
Israel said its Lebanon offensive was in response to Hezbollah's ongoing cross-border rocket attacks, which had displaced thousands of Israelis in the country's north.
Several high-ranking commanders and officials from the militant group have been killed.
And Hassan Nasrallah, the group's longtime leader, was assassinated in an Israeli assault in September.
"After the heavy blow that Hezbollah suffered at the hands of Israel, the fall of Assad is a fatal strike on Iran's influence efforts in the Middle East," Mr Citrinowicz said.
"There is of course a connection between the two, since it is clear that the weakness of Hezbollah and especially the elimination of its leader Hassan Nasrallah, who was personally committed to saving Assad, accelerated the overthrow of the Syrian regime."
Iran itself has also become increasingly vulnerable.
Several influential Islamic Revolutionary Guard Corps officers have been killed in air strikes in Damascus and Beirut in recent months.
And a wave of Israeli strikes on Iranian military sites in October disrupted Tehran's ability to produce key weapons and took out many crucial air defence assets, according to the US Institute for the Study of War (ISW).
Axis failed to back Assad
The rebel militant group Hayat Tahrir al-Sham (HTS), also known as the Syrian Rebels, captured the capital, Damascus, from the Assad regime this week, bringing an end to Syria's 13-year civil war.
The group said it was now establishing a "salvation government".
Early in Syria's civil war, Iran and Hezbollah had sent in fighters when it appeared Assad might be overthrown — in the name of defending Shiite shrines in Syria.
Russia later joined with a campaign of air strikes.
The campaign won back territory, even as Syria remained divided into zones of government and insurgent control.
But the speed of Assad's collapse over the past week indicated just how reliant he was on support from Iran and Russia — which, at the crucial moment, didn't come.
Russia remains mired in Ukraine years after launching a full-scale invasion there in 2022.
James Gelvin, professor of history at the University of California, Los Angeles (UCLA), said because the Axis of Resistance had been at "almost a complete collapse", Iran had its own problems to deal with.
"And all the other members of the Axis that Syria has depended upon have been severely damaged or weakened," he told ABC's The World Today.
"This is why the military operation was pretty much so quick."
What's next for the alliance?
Mr Citrinowicz said the collapse of the Assad regime showed Iran's limited ability to influence what happened in the Middle East without Hezbollah by its side.
"Now Iran will have to calculate a new course and find a solution that will strengthen its ability to deter Israel and the United States on its own, with no real support of its proxies," he said.
For Israel, breaking Iran's regional network has been a major goal.
Prime Minister Benjamin Netanyahu called Assad's fall a "historic day," saying it was "the direct result of our forceful action against Hezbollah and Iran, Assad's main supporters".
Dr Mamouri said Iran's regional influence had been severely diminished, leaving it more vulnerable to direct conflict with Israel.
But the Axis of Resistance was unlikely to fall apart.
"It has very strong organisation, it has very strong staff and fighters, it has a very workable operation system, and it has a strong hierarchy system," he said.
"So it's unlikely to say that it will fall down.
"But it will face significant challenges in terms of their political presence in both Syria in general and the Lebanese government, and also in terms of military resistance against Israel in particular."
Iran will struggle to rebuild Hamas and Hezbollah, so analysts believe Iran will likely prioritise shifting the centre of gravity of the Axis of Resistance eastward.
Dr Mamouri said Iran would be able to draw on its connections to Yemen and Iraq, and their armed groups, including the Popular Mobilization Forces (PMF).
The size of the PMF is heavily contested, with estimates ranging up to 200,000 fighters.
"I think they [Iran] are still in the situation of processing the events, but I don't think they are out of options," he said.
Iran's nuclear threat
Iran still holds the card of its nuclear program, although it insists it only enriches uranium for peaceful purposes.
The head of the International Atomic Energy Agency last week warned that Iran was poised to "quite dramatically" increase its stockpile of near weapons-grade uranium.
Its nuclear program now enriches uranium at 60 per cent, a short, technical step from weapons-grade levels of 90 per cent.
A recent US intelligence community report stated that "Iran is not building a nuclear weapon" but it had "undertaken activities that better position it to produce one, if it so chooses".
Malcolm Davis, senior analyst at the Australian Strategic Policy Institute (ASPI), said Iran would likely be worried about its own stability, and focus on preventing civil uprising inside its own country.
But there was also the threat of nuclear escalation in response to Israel's "much stronger position".
"It needs to somehow redress the imbalance that's now emerged as a result of the Axis of Resistance seemingly falling apart, so it could race for nuclear weapons options as well," he said.
"[US] President Biden or president Trump would need to pack very swiftly to prevent that from happening."
However, Dr Mamouri said the Iranians would know where to draw the line.
"I think they will deal with the ongoing, current and future situation with wisdom, but at the same time with resistance," he said.
"They don't act crazy. They don't act without strategy."
ABC/AP
