Syria is of vital importance to the Tehran regime, not least because it's the conduit for missiles and other weapons to Lebanon's Hezbollah, Iran's proxy in the Levant and its spearhead against Israel, as well as the Palestinian Hamas movement in the Gaza Strip.
It is also the gateway for Iran's plans to expand its influence westward into the Arab world all the way to the Atlantic and bolster its growing influence in Arab affairs.
If the Syrian regime collapses it would be a major geopolitical setback for Iran and for Hezbollah.
Syria's alliance with Shiite Iran, forged by Assad's father in 1980 at the start of the eight-year Iran-Iraq war, has never sat well with the Sunni-dominated Arab world and its demise would be applauded in most Arab capitals.
U.S. officials in recent days have confirmed reports that Iran is providing equipment and training to help Assad crush widespread opposition to the Damascus regime that began March 15 and in which about 900 people have been killed.
Both Tehran and Damascus are notorious for their secretiveness, so the reports are difficult to verify since Damascus bars foreign journalists.
"There is no smoking gun yet," Israeli analyst Jonathan Spyer wrote in The Jerusalem Post May 21. "But the circumstantial evidence is accumulating and the variety of sources from which it is emanating point to there being at least something to it."
The Iranians have every reason to want to help Assad hold onto power because of the foothold it gives the Islamic Republic in the eastern Mediterranean right up to Israel's northern border.
If the Assad dynasty, dominated by the minority Alawite sect, which has ruled Syria with an iron hand since 1970, is brought down the Iranians would likely face a new regime led by the majority Sunnis who would almost certainly severe the alliance with Tehran, a major strategic setback for Iran.
"Syrian regime survival would be a glowing advertisement to regional leaders that, unlike the U.S., Iran will do all it can to keep its friends from overthrow," Spyer observed.
Iran has long had a military presence in Syria, largely a contingent of the al-Quds Force, clandestine arm of the powerful Islamic Revolutionary Guards Corps. This force, based at Zabadani, the IRGC's main support facility for Hezbollah, is several hundred strong.
Whether those personnel are involved in operations against Syrian protesters is unlikely, given their mission.
But U.S. officials and Arab diplomats say other al-Quds operatives have been deployed for that purpose, training Syrian forces in how to contain the kind of large-scale street protests that the Tehran regime has in recent years put down with overwhelming force.
The Syrians' recent move to carrying out wide-ranging mass arrests of suspected dissidents and troublemakers reflects tactics used by the IRGC and its militia, the Basij, in Iranian cities.
In the last few weeks, Syrian forces, primarily the widely hated Mukhabarat, or secret police, have rounded up an estimated 10,000 people, a tactic they haven't employed before.
Until the uprising began in March, Syrian authorities had little experience in dealing with mass street protests.
Iran also appears to have supplied Syria with sophisticated electronic equipment to monitor Facebook and Twitter to identify protest organizers and their supporters, just as Tehran did during mass protests following the 2009 presidential elections.
These tactics presumably led to the recent wave of mass arrests, with up to 10,000 people seized, human rights groups claim.
In recent days, there have been persistent reports that Brig. Gen. Mohsen Chirazi, considered to be the third-ranking leader in the al-Quds Force, is in Syria.
The U.S. administration included Chirazi in a recent list of key figures in the Syrian regime it hit with sanctions over the violent crackdown.
Chirazi, a heavyweight clandestine operator, was captured in Baghdad by the U.S. troops in December 2006 for allegedly organizing Iraqi insurgents and supplying them with Iranian arms but was later released.
His reported presence and Iran's increasing presence in Syria underlines Tehran's growing anxiety about the prospect of Assad's falling from power.