Iran spent decades building the so-called axis of resistance, its network of regional armed proxies, Tehran-backed militant groups, and allied state actors.
The network was the lynchpin of Tehran's efforts to deter Israel and the United States and exert its influence across the Middle East.
But the fall of the government of Syrian President Bashar al-Assad, an ally of Tehran, has done irreparable damage to the network, analysts say.
For Iran, Syria provided a crucial land corridor to the Levant that was considered the logistical backbone of the axis. The corridor, also known as the Shi'ite Crescent, connected Tehran to the Lebanese armed group Hezbollah, a key ally and an integral part of the axis.
"There is no axis without access," said Ali Vaez, director of the Iran Project at the Brussels-based International Crisis Group. "The resistance is not done, but losing the ability to logistically support Hezbollah means the loss of Iran's strategic depth."
Broken Corridor
Underscoring Syria's importance, Iran spent billions of dollars to keep Assad in power.
Tehran intervened militarily in Syria's civil war in 2013 and played a key role in shoring up Assad's forces. It deployed hundreds of Islamic Revolutionary Guards Corps (IRGC) officers to recruit and train tens of thousands of local and foreign Shi'ite fighters.
Iranian Supreme Leader Ayatollah Ali Khamenei meets Syria's then-president, Bashar al-Assad, in Tehran in February 2019.
After the loss of the land corridor connecting axis members from Iran to Lebanon, "we are likely to see a much-diminished resistance in the coming months and years," said Farzan Sabet, senior research associate at the Geneva Graduate Institute.
The axis, he said, will have "a considerably lower capacity to rebuild or conduct military operations in the future."
At its height, the axis was active in the Palestinian Occupied Territories, Lebanon, Iraq, Syria, and Yemen, and was meant to give Iran the ability to hit its enemies outside its own borders while allowing it to maintain a position of plausible deniability.
But the axis has suffered a series of debilitating setbacks in recent months.
Syria is now effectively ruled by the U.S.-designated terrorist organization Hayat Tahrir al-Sham (HTS) and its allies -- some of whom are linked with Turkey, Iran's rival. The HTS seized power in Damascus on December 8.
Syrian opposition fighters celebrate after the Syrian government collapsed in Damascus, December 8.
Hezbollah has been severely weakened after a bruising, yearlong war with Israel, which killed the group's longtime leader, Hassan Nasrallah.
Meanwhile, Israel's devastating war in the Gaza Strip has diminished the capabilities of U.S.-designated Palestinian terrorist organization Hamas, another axis member.
Events in Syria "will certainly place significantly more restraints on Iran’s ability to maintain its regional influence," said Raz Zimmt, senior researcher at the Israeli-based Institute for National Security Studies.
Iran's 'Very Bad' Options
Weakened regionally, Iran now has tough decisions to make, including reconsidering its deterrence strategy and possibly developing a nuclear bomb, experts say.
Zimmt says Iran has two "very bad options" -- to do nothing and recognize that its deterrence against Israel has been compromised, or weaponize its nuclear program and expose itself to the possibility of an Israeli attack.
Things look equally grim for Iran’s axis of resistance, according to Sabet, who says Tehran will be under pressure to pull back its regional activities.
Sabet says Iran will seek to exploit any potential chaos in the region, including in post-Assad Syria, to reassert its influence.
"If the civil war in Syria is not quickly settled and a new order created, it might become precisely the type of environment where the Islamic republic has historically thrived," Sabet said.