Correction: An earlier version of this story incorrectly said that President Obama was expected to host a White House meeting on Syria on Wednesday. The president is not expected to attend the meeting. The story has been updated.

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

As fighters with Lebanon’s Hezbollah movement wage the battles that are helping Syria’s regime survive, their chief sponsor, Iran, is emerging as the biggest victor in the wider regional struggle for influence that the Syrian conflict has become.

With top national security aides set to meet at the White House on Wednesday to reassess options in light of recent setbacks for the rebels seeking Syrian President Bashar al-Assad’s ouster, the long-term outcome of the war remains far from assured, analysts and military experts say.

But after the Assad regime’s capture of the small but strategic town of Qusair last week — a battle in which the Iranian-backed Shiite militia played a pivotal role — Iran’s supporters and foes alike are mulling a new reality: that the regional balance of power appears to be tilting in favor of Tehran, with potentially profound implications for a Middle East still grappling with the upheaval wrought by the Arab Spring revolts.

“This is an Iranian fight. It is no longer a Syrian one,” said Mustafa Alani, director of security and defense at the Dubai-based Gulf Research Council. “The issue is hegemony in the region.”

The ramifications extend far beyond the borders of Syria, whose location at the heart of the Middle East puts it astride most of the region’s fault lines, from the Israeli-Palestinian conflict to the disputes left over from the U.S. occupation of Iraq, from the perennial sectarian tensions in Lebanon to Turkey’s aspirations to restore its Ottoman-era reach into the Arab world.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

An Iran emboldened by the unchecked exertion of its influence in Syria would also be emboldened in other arenas, Alani said, including the negotiations over its nuclear program, as well as its ambitions in Iraq, Lebanon and beyond.

“If Iran wins this conflict and the Syrian regime survives, Iran’s interventionist policy will become wider and its credibility will be enhanced,” he added.

From Iran’s point of view, sustaining Assad’s regime also affirms Iran’s control over a corridor of influence stretching from Tehran through Baghdad, Damascus and Beirut to Maroun al-Ras, a hilltop town on Lebanon’s southern border that offers a commanding view of northern Israel, according to Mohammad Obaid, a Lebanese political analyst with close ties to Hezbollah.

Iran has sought to minimize its visible involvement in Syria so as not to exacerbate sectarian tensions that have been inflamed by a conflict pitting an overwhelmingly Sunni opposition against a regime dominated by Assad’s minority Shiite-affiliated sect, Obaid said.

Iran has provided advice, money and arms to Assad’s regime, but the manpower needed to bolster his forces, flagging after two years of trying to contain the revolt, has come from Hezbollah, which was founded in the 1980s with help from Iran’s Revolutionary Guard Corps and has become Lebanon’s leading military and political force.

“Hezbollah is part of the Iranian strategy,” Obaid said. “This counts as a victory for the group of Iran, Syria, Iraq and Hezbollah against the group backed by the United States.”

‘Iran walked the walk’

Supporters of the Syrian opposition contrast the hesitancy of the U.S. administration in offering arms to the outgunned, poorly trained and deeply divided rebels with the commitment that Iran has shown to its Damascus ally.

The U.S. goal was to pressure Assad into making concessions at the negotiating table, without delivering a resounding military victory to the rebels that might have brought Islamists to power in Damascus, said Amr al-Azm, a history professor at Shawnee State University in Ohio who is Syrian and is active in the opposition. Instead, a proposed peace conference in Geneva seems likely to be held on Assad’s terms, should it go ahead.

“Politically we’re screwed, and militarily we’re taking a pounding,” Azm said. “America talked the talk while Iran walked the walk.”

This would not be the first time that Iran has outmaneuvered the United States since the Iranian revolution brought Shiite clerics to power in Tehran in 1979. But the assertion of Shiite power in Syria rankles Sunnis across the region, compounding the dangers that the Syrian conflict could provoke a wider and even bloodier war than the one currently underway, which is estimated to have killed at least 80,000 people.

Escalating violence in Iraq and growing tensions in Lebanon, whose conflicts are inextricably intertwined with the increasingly sectarian nature of the war in Syria, underscore the risk that centuries-old religious rivalries between Sunnis and Shiites will be aggravated by Iran’s role. The leading religious authority in Saudi Arabia and al-Qaeda chief Ayman al-Zawahiri have in the past week called on Sunnis to volunteer to fight in Syria, marking a potentially dangerous convergence that could herald an intensified influx of Sunni jihadis.

Saudi Arabia’s role

Saudi Arabia, the leading Sunni power in the region and Washington’s closest Arab ally, is unlikely to tolerate an ascendant Iran even if the United States chooses to remain aloof, said Jamal Khashoggi, director of the al-Arab television channel.

“It is a serious blow in the face of Saudi Arabia, and I don’t think the Saudis will accept it. They will do something, whether on their own or with America,” he said. “Syria is the heart of the Arab world, and for it to be officially conquered by the Iranians is unacceptable.”

One way in which Saudi Arabia could influence the outcome is by facilitating unchecked supplies of arms to the rebels, analysts say. Although the umbrella Free Syrian Army has received small quantities of weaponry from Turkey, Saudi Arabia and Qatar over the past year, the United States has sought to control the flow, vetting the recipients and restricting the caliber of the weapons provided.

After videos surfaced in March of Islamist groups wielding antitank weapons funneled across the Jordanian border by Saudi Arabia, the United States imposed a freeze on all further deliveries, putting the rebels at a disadvantage just as Iran, through Hezbollah, was gearing up to rejuvenate the Assad regime’s army with reinforcements, according to rebel leaders.

A symbolic battle

Military analysts caution against overestimating the impact of the rebel defeat in Qusair on what is likely to be a long and unpredictable war. The obscure western town abutting Hezbollah-controlled territory in Lebanon almost certainly offered an easier conquest than other rebel strongholds, such as the city of Aleppo, where the regime is touting an imminent offensive.

The rebels are continuing to press attacks in the northern, eastern and southern peripheries of the country even as the government appears to be tightening its grip on the central provinces of Damascus and Homs, raising the specter that the country will be partitioned into enclaves backed by rival Sunni and Shiite regional powers. A suicide bombing in Damascus on Tuesday highlighted the likelihood that the rebels will sustain an insurgency similar to the one that persists in Iraq even if they are defeated militarily.

The chief significance of the battle for Qusair lay in the powerful symbolism of the role played by Hezbollah, which eliminated any doubt that the Syrian conflict has turned into a proxy war for regional influence, said Charles Lister, an analyst with IHS Jane’s defense consultancy in London.

“External actors are becoming increasingly decisive and pivotal in terms of where the conflict is going,” he said. And if the United States increased its support for the rebels, Assad’s allies would be likely to boost theirs, he added.

“The conflict has regionalized, and, unfortunately, that gives it the potential to drag on longer,” he said. “As long as one side increases its assistance, the other will see the need to do so, too.”