Part of a series of articles looking at the regional interests at stake in Syria's civil war. The full list is on the left of your screen.

As Syria’s closest regional ally for decades, Iran is doing all it can to prevent the fall of President Bashar al-Assad. Iran’s Supreme Leader Ayatollah Ali Khamenei has warned that “fire” will engulf the Mideast if US forces strike.

Senior Iranian officials consider Syria the indispensable center of the Iran-led “Axis of Resistance” to US and Israeli influence in the Middle East, a key strategic bridge that for three decades has served as a link to the frontline with Israel and as a conduit to arm and support Hezbollah and Palestinian militant groups.

For Iran the stakes could not be higher in Syria, where – by deploying military advisers and permitting Hezbollah fight on Assad’s behalf against US-supported rebels – Iran feels it is engaged in a proxy battle against what it calls the “global arrogance.”

The best outcome for Tehran is an Assad victory that ensures Iran keeps its bridgehead to the Arab world and its fighting allies. The worst outcome for Iran is a toss-up: either a toppling of the regime by American-backed rebels or by the jihadi Islamic fighters on the rebel side, who despise Iran and the US in equal measure.

Clinging to Syria has created policy dilemmas for Iran, which has cast this popular uprising not as a part of the welcomed “Arab Spring” that saw dictators brought down in Tunisia, Egypt and Libya, but as a fight instead against terrorism.

Also a problem: Assad’s alleged use of chemical weapons repeatedly against his own people. Iran is strongly opposed to chemicals weapons use by any side, a policy stemming from being victimized by Iraqi chemical strikes in the 1980s.

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Every day, powerful rhetoric supporting Assad comes from Tehran, with florid threats aimed at those who would do Syria harm, issued most often by hardliners.

In one example, a leader of Iran’s Basij paramilitary group declared: “In case of America’s potential invasion of Syria, Basiji seminary students are eagerly prepared to stand up against the enemies and this confrontation will doubtless be their honor.”