• Syria's embattled president is losing ground to advancing rebels after years of stalemate.
• In the past, Iran orchestrated the interventions that stopped the opposition forces' momentum.
• "I'm not sure Iran can muster the numbers in time to turn this situation around," an analyst said.

Since the early stages of Syria's bloody civil war that began in 2011, Iran has supported Syria's strongman president, Bashar Assad. Now with the loss of Syria's second city, Aleppo, to Assad's opponents, Tehran has vowed to continue this support — but it almost certainly has less to offer than a decade ago.

Syrian rebel forces spearheaded by the Islamist Hayat Tahrir al-Sham militant group seized Aleppo in a shock offensive last week, surprising the world and returning the Syrian conflict to the headlines for the first time in years. The rebels did not stop there. Advancing in the face of Russian and Syrian airstrikes, they overran Syria's fourth-largest city, Hama, on Thursday.

"The Syrian government has lost a lot of terrain, including the city of Aleppo. You can't really overstate the seriousness of that," Aron Lund, a fellow with Century International and a Middle East analyst at the Swedish Defence Research Agency, told Business Insider. "Aleppo is a huge city, a really large urban area that will be difficult to retake once lost if Assad is unable to move on it before the insurgents dig in."

Earlier in the Syrian conflict, Iran helped orchestrate interventions on Assad's side by its powerful Lebanese proxy Hezbollah in 2013, and Russia in 2015.

These decisive interventions helped him turn the tide, culminating in a ferocious, scorched-earth campaign against opposition groups in east Aleppo in 2016.

The latest offensive has prompted some outside intervention. Hundreds of Iran-backed militiamen in Iraq are entering Syria. Russia has carried out airstrikes to impede the opposition's advance. However, these are small-scale compared to past interventions. And Hezbollah isn't intervening for now.

"I'm not sure Iran can muster the numbers in time to turn this situation around," Lund said. "Hezbollah, which was Tehran's primary instrument in Syria over the past decade, is now stuck in Lebanon, tending to its wounds and trying to get back on its feet after being mauled by Israel over the course of a two-month war."

Hezbollah is estimated to have upwards of 100,000 fighters and a vast missile arsenal, but those have been battered by Israel's airstrikes and occupation of southern Lebanon.

"The Lebanon ceasefire is really brittle, and as long as conflict could re-erupt at any moment, I don't think Hezbollah has the manpower to spare," Lund said. "Even if they're able to send some men Assad's way, I doubt it would be a game-changing number."

Iran's other options are mobilizing more Iraqi militias or sending personnel from its own Islamic Revolutionary Guard Corps paramilitary or its regular army.

"It's possible that Iran will try to send more arms and ammunition," Barbara Slavin, a distinguished fellow at the Stimson Center in Washington and founder and former director of the Future of Iran Initiative at the Atlantic Council, told BI.

"It is also probably counting on Russian airstrikes to stall the HTS offensive and it is clearly having back-channel talks with Turkey," Slavin said. "The Turks may be angling to get Iran to turn a blind eye to new attacks on the Kurds in return for urging HTS to hit pause."

HTS is not a Turkish-controlled proxy like the self-styled Syrian National Army coalition of opposition militias that is also advancing across Aleppo province. However, Turkey has closely coordinated its army's deployment in Syria's Idlib with HTS, which has been the predominant power in that northwestern Syrian province for years now.

"Iran is also looking to cultivate a relationship with Trump, which further limits its freedom of maneuver in the region," Slavin said.

"Iran is in a terrible situation currently without the necessary military, economic, and political capacity to spare," Arash Azizi, senior lecturer in history and political science at Clemson University, told BI.

"Its myriad of problems includes the fact that many Iraqis are reluctant to get involved in a renewed war in Syria."

While Iran will undoubtedly remain committed to supporting Assad, it's unlikely to prove capable of organizing a 2016-style counteroffensive to recapture Aleppo.

"That would require a lot of planning and a lot of diplomatic bargaining with Turkey and other powers," Azizi said.

Century's Lund recalled that the last time Iran intervened to help Assad recapture Aleppo, it did so in close collaboration with Russia.

"If they're going to replicate that success now, the Russians would probably need to bring the air component," Lund said. "Iran has no air force to speak of and if Iranian jets were to show up in Syria, Israel would go after them immediately."

Russian jets and air defenses afford Iran some protection since Israel is more hesitant to clash with Russian forces than they are with Syrian or Iranian ones.

But it remains to be seen how much airpower Russia can spare with its fighters and bombers engaged in the invasion of Ukraine.

"It's worth noting that the Syrian conflict is so small-scale compared to Ukraine that even a relatively small contribution of assets could have an impact there," Lund said.

The Iraqi militiamen entering Syria are more likely to serve as a holding force to help Assad avoid losing more territory.

"Assad needs to hold onto the capital and its immediate environs if he has a chance to survive as Syrian leader," Stimson's Slavin said.

Since Syria doesn't have a sizable Shia minority like Lebanon, Iran has been unable to stand up a local proxy as effective or powerful as Hezbollah in neighboring Lebanon.

Any efforts Iran may take to help Assad coincide with its own priorities to seek an acceptable deal with President-elect Donald Trump's incoming administration. "Iran also wants to at least try for a deal with Trump, so a more aggressive regional posture will not work as regional issues will be on the table this time along with the nuclear file," Slavin said.

As this crisis unfolds, it's striking how "unimportant and absent" the US has been, Azizi noted.

"The US has forces on the ground and also airpower that it has used to attack certain forces on Syrian territory," Azizi said. "But it's clearly not a main player and doesn't seem to have a clear, strategic goal or any particular focus on Syria."

And it remains unclear what President-elect Trump will do about Syria upon reentering office in January.

"As with most other matters, President Trump remains unpredictable," Azizi said.

Paul Iddon is a freelance journalist and columnist who writes about Middle East developments, military affairs, politics, and history. His articles have appeared in a variety of publications focused on the region.