When the Russian Air Force intervened in Syria on September 30, 2015, it changed the tide of battle. After a year of painful defeats in places such as Idlib, Jisr al-Shughour, Palmyra, and the Hawran region, Syrian President Bashar al-Assad’s government managed to regain its balance. By the end of the year, Assad’s forces were again moving forward in the northern Latakia region, east of Aleppo, and on several other fronts. In February 2016, his army cut a key rebel supply route between Aleppo and the Turkish border, and, in late March, Assad’s Russian-backed troops retook Palmyra from the self-proclaimed Islamic State. They are now moving on the Ghouta enclave east of Damascus, exploiting weeks of disastrous infighting among the local rebels.

Assad’s advances have slowed down recently, partly due to a brittle cessation of hostilities agreement monitored by the United States and Russia. The government even lost some ground in the Aleppo, Latakia, and eastern Homs regions. More significantly, the Syrian economy is in disastrous shape, and this might undermine Assad’s military progress. But there is no question that Assad’s position has greatly improved due to the Russian intervention, or that Moscow’s influence over the conduct of the war in Syria has grown significantly.

That is the conventional narrative, at least. However, it is missing something.

What happened in autumn 2015 was not just that Russia began operating in Syrian airspace. The reason the Russian intervention was so successful was that it was also accompanied by Iranian intervention on the ground. Let’s take a closer look at how that happened.

The Growing Iranian Presence in Syria

Iran has been allied to the Assad family in Syria since 1979, in a curious but close relationship that has evolved over time. In its early phases, the relationship was balanced in Syria’s favor, but by the 2000s, this had changed. Nevertheless, Iran’s supreme leader, Ali Khamenei, has made no secret of his view of Syria as an indispensable regional ally. That is partly because the Assad regime has facilitated Iranian influence in Lebanon, where the pro-Iranian Shia Islamist group Hezbollah is based. This helps Iran to balance and contain its militarily superior archenemy, Israel. As Khamenei’s foreign policy adviser Ali Akbar Velayati put it in 2012, “Syria is the golden ring of the chain of resistance against Israel.”

When the current conflict erupted in 2011, Iran stepped in with strong support for Assad, but it initially preferred to act behind the scenes. Early forms of support included providing technical assistance, specialized equipment, and military training. Later, Iran would organize a lifeline of economic support, including Iraqi oil shipments and billions of dollars in loans and credits. Starting in 2012, Tehran also helped create pro-government militias known as the National Defense Forces.

At the same time, with Iranian encouragement and backing, Lebanon’s Hezbollah began to train the Syrian army and offer other forms of specialized assistance as early as 2011. The group began playing an overt combat role in spring 2013.

By contrast, Iranian citizens were not significantly involved in the fighting at the time. To be sure, there was a small-scale presence of officers from Iran’s Islamic Revolutionary Guards Corps (IRGC) and the Quds Force. The IRGC, which has ground, air, and naval branches, is organizationally distinct from the regular Iranian army, known as Artesh. It answers directly to Ayatollah Khamenei. The Quds Force—Quds means Jerusalem—is an elite intelligence and special forces branch. It is formally part of the IRGC, but bypasses the chain of command to instead report directly to Khamenei. The IRGC and Quds Force officers mainly seem to have been present in an advisory role, to train and support Shia militias and to some extent the Syrian army. Nevertheless, Iranians were occasionally reported dead in the first few years of the war, particularly after the IRGC began to play a more active role in 2014. But it was not until autumn 2015 that casualty numbers really began to rise.

The Russian-Iranian Intervention in 2015

Russian air raids in Syria began on September 30, 2015. By that time, it appears, Iran had already started airlifting Shia fighters into Syria from Iraq. Along with them came, for the first time, significant numbers of Iranian nationals. According to one report, Iran is believed to have boosted the IRGC contingent in Syria from some 700 military advisers to approximately 3,000. Meanwhile, Iranian media began a propaganda campaign intended to persuade the population that an intervention in Syria would be morally just and in keeping with Iranian national interests. (The same thing happened in Russia, with reporters and resources from state-owned propaganda outlets suddenly shifting away from Ukraine toward Syria.)

Within two weeks of the first Russian airstrikes, Iran unleashed a ground offensive south of Aleppo. Non-Syrian Shia militias dominated the offensive, but they were seemingly led, coordinated, and reinforced by small groups of IRGC personnel. By spring 2016, Syrian and foreign Shia fighters backed by Russian airpower managed to break through rebel lines north of Aleppo, relieving the isolated Shia enclaves of Nubl and Zahra and cutting rebel supply lines to Turkey—a major breakthrough and a direct result of the joint Russian-Iranian surge in Syria.

The increased level of Iranian involvement can be documented by studying the casualty rates from October 2015 onward, as done by researchers at the Washington Institute for Near East Policy and the American Enterprise Institute, two American think tanks (both of which are generally associated with hardline policies against Iran). According to statistics compiled from open sources, the number of dead Iranian nationals in Syria shot up dramatically beginning in autumn 2015. All in all, some 160 Iranians were killed and 300 wounded between October 2015 and February 2016, nearly doubling the number of Iranians who have died in Syria since 2011. The casualty lists also included a larger number of mid- and low-ranking officers, in addition to senior officers like those who had occasionally been killed early in the war. While colonels and major generals would presumably have been present in leadership roles or to organize training programs, lieutenants and captains are more likely to have been killed while leading troops on the battlefield.

In for the Long Haul

With the winter offensive’s achievements and Russian-American truce discussions moving toward implementation, Iran seems to have recalled some of its troops. While this may have been part of a normal rotation of forces, it appears to have been presented as a more permanent retreat and accompanied by discreet Iranian signals of respect for the cessation of forces.

Indeed, many Western governments interpreted the troop transfers as Iran returning to its pre-intervention posture. For example, U.S. Secretary of State John Kerry told a U.S. congressional hearing on February 26 that “the IRGC has actually pulled its troops back from Syria.” Ayatollah Khamenei “pulled a significant number of troops out. Their presence is actually reduced in Syria,” said Kerry, as reported by the AP.

However, on March 9, a leading IRGC commander denied that there had been a suspension of the troop deployments to Syria. Two weeks later, the regular Iranian army announced that it, too, was training commando units for Tehran’s intervention in Syria. Soon thereafter, army units had deployed on the ground, taking their first casualties on April 11. In early May, Velayati met Assad in Damascus, where he said that Iran would “mobilize all resources to fight the terrorists that are perpetrating crimes against oppressed nations in the region regardless of the ridiculous categorization of those terrorists as moderates and extremists.”

The Kremlin seems to have used the cessation of hostilities to nudge Assad eastward, urging him to focus on fighting the Islamic State. But Iran and Hezbollah appear to have maintained their focus on Aleppo and other areas of more critical importance to Assad’s survival.

As the Russian- and U.S.-backed truce has faltered, fighting has flared up. Although sometimes engaged in a defensive role, Iranian-led Shia units have been at the center of this renewed violence—and they have to be. If the Syrian army were to be left on its own without Iranian and Shia militia backing, it is unlikely that it could hold the front lines carved out in spring 2016, even with Russian airpower on hand for support.

Indeed, if casualty reports indicate of how invested Iran is in Assad’s survival, Tehran seems to have increased its commitment. One open-source tally counted 75 deaths in March and April 2016, and on May 7 alone Iranian media reported 13 Iranian “military advisers” killed and 21 wounded south of Aleppo. If anything, the Iranian involvement seems to be rising.

Tehran’s deepening commitment to saving Assad’s government needs to be taken into account when analyzing Western, Arab, and Russian strategies in Syria. However much Russia and Iran may disagree over the Geneva III talks, it is clear that Iran has by now come to play a key role on the ground in Syria—and that both Damascus and Moscow depend on a continued presence of Iranian combatants to ensure the success of their own strategies in Syria.