Iran's military support to Bashar al-Assad is starting to have a decisive impact on the course of Syria's brutal sectarian conflict. For the first time in more than two years of fighting, in which roughly 90,000 Syrians have perished, the Assad regime now appears to have a distinct chance at survival—a remarkable turnaround for a government that, less than a year ago, looked doomed.

Recent advances by Assad forces against rebels on the outskirts of Damascus, Aleppo and the Lebanese border town of Qusayr—where the regime has been engaged in a bitter battle to reclaim control of the city—suggest that the tide of the conflict is shifting dramatically in favor of Assad.

These recent successes contrast starkly with the regime's perilous position last July, when former Defense Minister General Dawoud Rajiha and other high-ranking security officials were assassinated in a suicide attack at the National Security headquarters in Damascus. Back then, it seemed only a matter of time before Assad lost his grip on power.

But looking back on the course of the conflict, last summer now seems to have been a turning point for Assad—not least because it persuaded Iran, Syria's long-standing regional ally, to intensify its military support for his regime.

Tehran regards the survival of the Assad regime in Damascus as essential to maintaining Iranian links with Hezbollah in Lebanon. So after the bombing at the National Security Headquarters, Iranian President Mahmoud Ahmadinejad ordered Revolutionary Guards commanders to increase the flow of Iranian weapons and supplies to Syria, as well as training. The Revolutionary Guards sent a 150-strong team of military instructors to Damascus in September, to help develop pro-Assad fighters into an effective military force.