Iranian officials have routinely stressed the importance of Syria in Iran’s regional designs. Some have called on Iran to “possess” Syria, while others have framed Iran’s intervention there as necessary “to protect the accomplishments of the Islamic Revolution.” A chief accomplishment of this revolution was the creation of Lebanese Hezbollah.

Syria remains Tehran’s preferred Iranian land conduit to arm Lebanese Hezbollah with rockets and missiles. These weapons are more than just an impediment to Israeli security. They are what Iranian officials are counting on when they routinely threaten to destroy Tel-Aviv and Haifa. Iran has also tasked Lebanese Hezbollah with turning the tide in Tehran’s favor in multiple conflict zones. In Iraq, the group refined the terrorist tradecraft of anti-American Shiite insurgents. In Syria, they fought alongside Assad’s army in countless battles. In Yemen, they are reportedly training the country’s Houthi rebels. The success Iran had with Hezbollah can only be matched by the recent proliferation of pro-Iran Shiite militias throughout the region.

To shore up Assad, Iran has created scores of these mini-Hezbollahs all under the guise of combatting the so-called Islamic State. In the battle to save Assad, they are complimented by members of Iran’s own Islamic Revolutionary Guard Corps (IRGC), as well as thousands of Afghan and Pakistani Shiites. Despite suffering casualties in Syria, all of these forces remain active. But the regime has not stopped at troops. As early as 2012, the Islamic Republic provided the Assad regime with powerful conventional munitions, like the Fateh-110 short-range ballistic missile. Since then, Iran has hedged against interdiction strikes—which the Israelis have executed multiple times—and has reportedly taken to building missile bases and factories on Syrian soil.

All of this should focus the mind of President Trump and his advisors on Iran’s activities in Syria. The threat posed by the Assad regime to the Syrian people, to Israel, and to broader Middle East stability (as well as to Europe through the prism of the refugee issue) remains a function of the larger threat posed by an ascendant Iran. The lack of a coherent response to Iran in the Syrian theater makes this task all the more urgent, and admittedly, difficult, for U.S. national security planners.

Last April, in response to the Assad regime’s use of sarin, President Trump attempted to restore American credibility in Syria through limited cruise missile strikes. Iranian officials were quick to condemn the action, but did not fundamentally alter their activity in Syria. Although the strike could be interpreted as a signal of resolve against Assad and Iran, there was no follow-up to solidify that impression. To date, Washington has not developed a strategy in Syria to contest Tehran’s gains.

Any prospective U.S. military action in Syria must see last April’s strike as an indicator of what not to do again. First, to stem Assad’s use of chemical weapons, Washington must destroy the bulk of his regime’s delivery systems that include military jets and helicopters. Then, Washington should move to deny Assad use of his conventional weapons, some of which, like artillery, were also used to deliver chemical weapons. Should Washington decide to escalate further, it can target Shiite militiamen, as well as Lebanese Hezbollah. These forces function as auxiliaries to Syrian military power, having participated in both battles and sieges on behalf of Assad and Iran.

To compliment such actions, Washington must also bring to bear an arsenal of non-military tools such as economic sanctions against the Assad regime’s murky domestic assets and foreign financiers. This requires more than designating those who buy and sell oil on behalf of the Syrian regime. It means further penalizing Iranian financial institutions that enable Assad’s war machine and fund Iran’s logistics in Syria.

Another tool is diplomatic messaging. The recent wave of protests in Iran scolded the Islamic Republic for putting the regime’s foreign clients ahead to its own people. Consistently highlighting the cost of Iran’s intervention in Syria, be it in terms of blood, treasure, or national reputation, might invigorate the population to return en-masse to the streets. It would also signal solidarity with the countless Iranians whose cries of “Abandon Syria, think about us” during the 2017/2018 protests were derided by pro-IRGC outlets.

To be clear, the U.S. recognizes the need to push back on Iran’s regional ambitions, but seems unwilling to apply this idea to Syria. In October 2017, President Trump unveiled a long-awaited and admirable integrated Iran strategy that framed the Iranian challenge as being much broader than just the nuclear issue. That strategy plainly stated: “The Iranian regime has taken advantage of regional conflicts and instability to aggressively expand its regional influence and threaten its neighbors with little domestic or international cost for its actions.”

The Trump administration must impose a significant cost on Iran for its regional destabilization and support to the Assad regime. As long as Washington circumscribes its involvement in Syria, it will fail to meaningfully impede Iran. “We are waiting for you in Syria,” taunted the head of a pro-Iran Shiite militia. Decisive action in Syria against Iranian interests would ensure these militias do not have to wait much longer.

Behnam Ben Taleblu is a research fellow at Foundation for Defense of Democracies (FDD), a Washington-based, nonpartisan research institute focusing on national security and foreign policy.