For decades, Iran has expended much blood and money in support of Syria’s president, Bashar al-Assad, helping him survive a civil war that threatened his dynastic rule. Iran operated military bases, weapons warehouses and missile factories in Syria, which it used as a pipeline for arming its militant allies across the region.

Now, just as Mr. al-Assad needs help to repel a rapid advance by rebel forces, Iran is heading for the exits. On Friday, the country started evacuating its military commanders and personnel, as well as some diplomatic staff, according to Iranian and regional officials.

It is a remarkable turnabout: Iran not only appears to be abandoning Mr. al-Assad, its closest Arab ally, but also relinquishing everything it had built and fought to preserve for 40 years in Syria, its main foothold in the Arab world.

With the rebels expected to advance soon on Damascus, Iran is unable to muster a defense of the Assad government after a damaging year of regional wars that began with the Oct. 7 attack on Israel by Hamas, an Iranian ally.

A collapse of Iran’s partnership with Syria would by all accounts reshape the balance of power in the Middle East. The “axis of resistance” that Iran has formed with its militant allies in Lebanon, the Palestinian territories, Syria, Iraq and Yemen would be weakened. Israel and its Arab allies would be strengthened.

“For Iran, Syria has been the backbone of our regional presence,” Hassan Shemshadi, an expert on Iran’s proxy militant groups who was for years a documentary filmmaker on the battlefields of Syria, said in an interview from Tehran. “Everything that Iran sent to the region went through Syria. It is now extremely difficult to keep these channels open.”