Iran has played a pivotal role in Syrian President Bashar al-Assad's campaign to crush rebel resistance in Aleppo and is now close to establishing a "Shia crescent" of regional influence stretching from the Afghan border to the Mediterranean Sea.
Revolutionary Guards commanders and senior clerics in Tehran have this week praised Iran's defeat of "Wahhabi terrorists" in Syria and the country they characterise as the rebels' patron, Sunni Muslim regional rival Saudi Arabia.
There is little doubt the capture of Aleppo after years of fighting, and at the cost of thousands of lives, will make Assad unassailable to the rebels who have sought to end his rule. It is unlikely such a victory would have been possible without Iran's steadfast supply of Shia fighters, money and weapons.
The central role the Shia majority nation has played, and the power it wields, was starkly illustrated on Wednesday when the evacuation of rebel-held districts was delayed after Tehran was said by opposition officials to have imposed new conditions.
Iran demanded a simultaneous evacuation of wounded from two towns besieged by insurgents, according to rebel and UN sources.
The civil war has pitted Assad, supported by Russia and Iran, against rebel groups backed by the US, Gulf Arab powers and Turkey. Iran's involvement over more than five years, first by providing military advisers and then by training and arming Shia militia, has not only helped shape the Syrian conflict, it has strengthened its own hand across the region.
Mediterranean
That is not only because of the imminent fall of Aleppo but also because of the gains the Shia-led government in Baghdad, a close ally of Tehran, has made in the battle to retake Mosul from IS terrorists. Thousands of Shia militia fighters trained by Iran are fighting on the side of the government in Iraq. Some have already fought in Syria to support Assad and pledged to go back if necessary.
Iran already has a great deal of influence in Lebanon, where it has deep historical ties with the Shia community and funds Hezbollah, the country's most powerful political and military movement, which is also fighting in Syria on behalf of Assad.
Establishing such a "Shia crescent" would give Tehran immense political clout in the region as it vies with arch-rival Riyadh and allow it to protect Shia communities in these countries. It would also present a military threat to Israel, through Syria and Lebanon, which Iranian officials regard as a deterrent to any Israeli aggression towards Iran.
For Saudi Arabia and other regional Sunni powers, increased Iranian power would come at cost to their own political, military and trading interests.
Existential threat
In turn, the Revolutionary Guards and Iranian Supreme Leader Ayatollah Ali Khamenei were able to sell the war to the Iranian public. Hardline news outlets close to the Guards have been printing stories about the existential threat posed by IS and the need to fight the group in Syria since Iran went public about its role in the conflict in 2012.
Earlier this year, Khamenei said that if Iran was not taking part in the war in Syria then they would be fighting the same enemy inside Iran.