President Donald Trump’s decision last week to militarily punish the regime of Bashar al Assad for using chemical weapons caught Syria’s closest ally Iran by surprise. Political and military leaders in Tehran are still uncertain whether the U.S. strikes were a one-time mission or represented a sea change in Washington’s policy in Syria. The Islamic Republic hopes it was the former, but there is a growing concern in Tehran that it may be the beginning of concerted efforts by Washington and its regional Sunni allies to topple Iran’s ally in Damascus. Due to this uncertainty, Tehran has so far limited its response to verbal condemnations and refrained from any retaliatory actions against the United States - although this may change in the future. Instead, the Islamic Revolution Guards Corps (I.R.G.C.) and its proxies in recent days have intensified cooperation with the Assad forces to target anti-regime rebel groups to project power and seize more territory. Tehran is also seeking to further strengthen its military ties with Russia by capitalizing on the widening rift between Washington and Moscow.

Iran’s Low-key Response

So far, the response from Iranian political and military leaders has been measured and calculated. Supreme Leader Ali Khamenei that the United States committed a “strategic mistake” by attacking Syria, and stressed the Islamic Republic would not “retreat from the field” in the face of U.S. threats. President Hassan and Foreign Minister Javad took to Twitter to condemn the attack, but did not threaten to walk away from the nuclear agreement or to take any retaliatory measures against Washington. Iran’s Friday prayer leaders were uncharacteristically mild in their criticism of Washington as well. This is because Tehran does not want to provoke a more aggressive U.S. response that could undo its latest gains in Syria and beyond.

The most serious warning came from Iranian Defense Minister Hossein Dehghan, who said more U.S. attacks against Syria “,” but he also did not say what Iran would do if Washington carried out more strikes.

One thing, however, is clear: The missile assault has not deterred Iran from continuing to aid the Assad regime. All indications in recent days show that the I.R.G.C. and its proxies will most likely double down on their support to Damascus, which will certainly cause more killing, displacement and destruction in the war-ravaged country. In the past five years, Iran has not only deployed its elite I.R.G.C. forces to help Assad but has also recruited, funded and trained tens of thousands of Iraqi, Lebanese, Afghan and Pakistani Shiites to defend Iran’s ally in Damascus. About half a million Syrians have died in the war, while millions more have become refugees in neighboring countries and Europe.

After the fall of Aleppo in December, Iranian leaders were celebrating their “victory” over the United States and its regional allies. But Thursday’s strikes have triggered concern in Tehran that Washington might increase its military engagement in Syria. Tabnak, an outlet affiliated with former I.R.G.C. commander Mohsen Rezaei, that the Trump White House wants to incrementally prepare the ground for a U.S. military intervention in Syria, the way the Bush administration did after the 9/11 attacks.

Closer Tehran-Moscow Cooperation

The U.S. missile strikes in Syria have also brought Russia and Iran closer, at least for now. On the diplomatic front, Tehran and Moscow condemned the attack, reiterated support for the Assad regime, and called for an international investigation to the chemical attack in Syria. On Sunday, Iran’s President Hassan Rouhani a phone conversation with his Russian counterpart Vladimir Putin and the two leaders vowed to further boost their cooperation in Syria. The foreign ministers of Iran, Syria and Russia are meeting in Moscow on April 14. Iranian and Russian defense ministers also held a phone conversation and reportedly agreed to intensify military operations against anti-Assad forces.

Separately, a joint operational command center of Russia, Iran and Iran-controlled militia groups in Syria said the U.S. strikes crossed “the red lines” and to jointly respond to any American “aggression” in the future.

Iran and Russia have cooperated closely to prop up the embattled Assad regime. During his July 2015 trip to Moscow, it was Iranian Quds Force Commander Qassem Soleimani who  the Kremlin to militarily intervene in Syria to save Assad – a development that rapidly changed the balance of power in Assad's favor and resulted in immense human suffering. Without the Russian air power and tens of thousands of militiamen recruited, trained and funded by Iran, the Assad regime would not have survived.

But recently there was a concern in Tehran that Moscow might make a deal with Washington and betray Iran in Syria. But the latest tension between Russia and the United States has provided Tehran an opportunity  to capitalize on to strengthen its ties with the Kremlin. Zarif said last month that the Islamic Republic would allow Russia to use Iranian bases “on a case by case basis” to launch air strikes against “terrorists” in Syria. The Russian military briefly used Iran’s Hamadan Air Base last August to for Syria operations, but the arrangement was canceled abruptly as a result of a backlash inside Iran after Moscow publicized the secret agreement. The Iranian military leaders may now try to convince the critics inside Iran that closer Russian-Iranian cooperation is needed to counter American actions in Syria. Tehran may also try to use the latest tension between Moscow and Washington to encourage the Russian government to sell more advanced weapons to Iran.

Will Iran Retaliate?

For now, Iran has adopted a wait-and-see approach. It is unlikely to escalate tension with Washington as long as the latter does not take further action to prevent the Syrian regime and its foreign allies from continuing their military operations. Tehran is also closely monitoring Moscow’s response to the U.S. strikes in Syria. Unless Russia decides to attack U.S.-backed rebels in Syria, Iran may not take the risk unilaterally.

But if Tehran feels its interests in Syria are threatened, it can retaliate against the United States both conventionally and asymmetrically. One Iranian outlet that the U.S. strike in Syria has provided Iran with “another opportunity” to retaliate against America in the region. Iranian naval forces could target U.S. warships and other military assets in the Persian Gulf. American military commanders have recently complained about an increase in Iranian hostile maritime actions in the region. But such a direct provocation is unlikely as it would draw a strong response from the U.S. military.

A more likely scenario would be that the I.R.G.C. tasks its regional proxies to attack U.S. military and civilian targets in the Middle East. With the Islamic State on the brink of defeat in Iraq, Iran-controlled Shiite militia groups have already dialed up a vicious propaganda campaign against American troops advising Iraqi security forces in Mosul and across the country. Iran’s Iraqi proxies are also pushing the Baghdad government to expel American troops from Iraq. Some of Iran-backed groups that are now fighting the Islamic State had been involved in deadly attacks against U.S. forces before. In Afghanistan, Tehran, perhaps in cooperation with Moscow, may further increase its support to the Taliban to harm the U.S. troops there. Read moreover, the I.R.G.C.’s elite Quds Force and Iranian proxies may also target U.S. allies in the region – particularly Sunni Gulf states and Israel. One Iran-sponsored Iraqi militia group recently announced the creation of a new brigade to seize the Golan Heights from Israel.

All this does not mean the U.S. and its allies should not push back against Iran’s subversive activities in Syria. Quite the oppostive, as long as Iran and its proxies dominate ground realities in Syria, a political settlement to end Syria’s six-year civil war is impossible. But the Trump administration and its allies must be ready to counter Iran’s potential retaliatory actions not only in Syria but also in the broader region.