Certain commentators have recently made the argument that Iran might back off should the U.S. decide to intervene in Syria. Up until now, it has been widely acknowledged that Iran has been supporting the Assad regime by providing diplomatic and logistical support. A U.N. panel reported in May that Iranian weapons destined for Syria but seized in Turkey included assault rifles, explosives, detonators, machine guns, and mortar shells.
Given the current situation however, if push comes to shove it is very unlikely that Iran will stand down if the U.S. decides to intervene in Syria. Put simply, Iran has way too much to lose should the U.S. intervene and quite a lot to gain should Assad's regime remain in power.
Iran's alliance with Syria is based on a number of factors that are of key national interest to Iran and the Khamenei regime. First and foremost is the part religion plays in the Iran-Syria alliance. Iran is the most populous Shiite Muslim nation and the Assad regime is dominated by Alawites, a Shiite offshoot. The Syrian rebels are Sunni.
Throughout its history Iran has battled and guarded against Sunni domination in the region. Its long standing animosity with Saudi Arabia, its war with Sunni Iraq, and its efforts to support the Shi'a community in Afghanistan are all indicative of the lengths Iran has gone and is willing to go to protect against Sunni domination in the adjoining areas. Similarly, the last thing the Iranian regime wants is a Sunni-rebel-dominated Syria. Some of these rebel groups have deep-rooted links with Al-Qaeda and Sunni Islamic extremism. According to Iranian national interests, it is vital for it to protect itself against these elements and continue to support Assad.
A U.S. intervention in Syria is also fairly detrimental to Iranian national interest from a strategic point of view. Syria is one of the very few allies Iran has in the region, which is dominated by countries which have substantial differences with Iran. Qatar, Saudi Arabia, Jordan, Iraq, etc. are all countries with which Iran enjoys a strategic and ideological rivalry. During the Iran-Iraq war, Syria was Iran's only ally in the region. Both countries have stood by each other over many decades.
In addition, the "we're next" factor also dominates Iranian apprehensions with regard to a U.S. intervention. A senior Iranian analyst pointed out to the Guardian recently that "National interests are national interests. If they take Syria, Iran is next, from the Iranian perspective. Iran hasn't been spending all these resources in Syria for nothing these past decades." The fall of Assad and a U.S. intervention will leave Iran strategically extremely vulnerable.
Also, invariably the Israel factor always plays a significant part in forming Iranian foreign policy preferences. The way Iran sees it, a U.S. intervention in Syria is meant to secure the region for Israel. This might be a rather unfounded theory considering that the biggest threat to Israel would come from the Al-Qaeda and militant Islamist groups among the rebels, rather than from Assad himself, who has a vested interest in not provoking his neighbors into intervention. However, Iran stands to lose its geographical access to Hezbollah in southern Lebanon if Assad falls. Syria acts as a conduit for the flow of weapons from Iran to Hezbollah. An armed Hezbollah presence in Lebanon acts as a bulwark against Israel, while equipping Iran with some insurance policy in case of an Israeli attack on its nuclear facilities.
Hence, given the Iranian priorities with regard to Syria and Assad it is highly unlikely that Iran will stand down in the case of a U.S. intervention. The entire region could face catastrophic repercussions should the U.S. decide to intervene and should Iran decide to respond in one way or another.