Iran has been a critical player in the Syrian war since 2011, crafting a complex foreign policy and military strategy to preserve its Syrian ally. What have been the drivers of Iranian decision-making in this conflict? And how has Iranian strategy evolved over the course of the war? This article argues that the logic of deterrence has been fundamental not just for shaping the contours of Iran–Syria relations since the Islamic Revolution of 1979, but also for determining the overall trajectory of Iranian strategy in the Syrian war. The authors outline Iran's decision-making calculus and divide the country's strategy on Syria after the Arab Spring into four primary phases: 1) a ‘Basij’ strategy to establish local militias in Syria; 2) a regionalization strategy to incorporate transnational fighters and militias in the war effort; 3) an internationalization strategy to incorporate Russia and balance the United States; and 4) a post-ISIS deterrence strategy to balance against the United States, Turkey and Israel. Iran's Syria strategy progressively escalated in response to the possible defeat of its ally and the deterioration of its forward deterrence capacities against the United States and Israel. Today, the potential for direct inter-state conflict is rising as proxy warfare declines and Iran attempts to maintain the credibility of its forward deterrence.

Interview with Hassan Ahmadian, a Postdoctoral Research Fellow at the Iran Project, Harvard Kennedy School Belfer Center for Science and International Affairs, to discuss topics such as the role of the U.S. in Syria and broadly in the MENA region; identity issues; and the role of Iran and the future of the nuclear deal.

Iran is primarily concerned with preserving Syria’s prominent role in the Axis of Resistance and its overarching goal of securing its regional deterrence. Accordingly, the collapse of Damascus was intolerable because it would have negatively affected Iran and its allies in the region. This includes securing supply routes to Lebanon; enhancing the deterrence capabilities and operational experience of the Axis of Resistance against Israel and the United States, especially with Trump’s renewed hostility; and balancing Turkey in northern Syria. In addition, preserving Syria’s significant position within the Axis of Resistance serves to showcase Iran’s effectiveness in supporting allies and in its leadership role in the axis.

Turkey is facing a new conundrum in Syria. While its main concern is the areas held by the Syrian Democratic Forces (SDF), due to the latter’s links with the Kurdistan Workers Party (PKK), Turkey has had to prioritize Idlib because of the looming attack on the province, which could jeopardize Ankara's presence in northern Syria. Iran, on the other hand, is prioritizing pushback against the United States east of the Euphrates. Tehran’s objective is to restore the status quo in Idlib — under Turkey’s influence — as well as northeastern Syria, where US forces are based. Against this backdrop, a strategic overhaul of Iranian-Turkish cooperation in Syria is not unprecedented.

Although there are a number of major disagreements between Iran, Russia and Turkey, what is important in the end is that they are all opposing the current US policies in Syria. All three countries have suffered from US sanctions and all have serious trust issues with the Trump administration. Emphasising this common ground, Iran's Supreme Leader Ayatollah Ali Khamenei told Russian President Vladimir Putin earlier in September that "the US is containable" and that Iran and Russia should continue close cooperation towards this goal. Whichever strategy Iran chooses to pursue, the US will likely feel the pressure on the ground soon enough. That could end up being an effective deterrent against US plans of opening additional fronts against Iran in the Middle East.

Having been targeted indirectly by the “leading from behind” policy under President Obama, Iran is now faced with a more organized US effort under Trump to establish a regional setting — an "Arab NATO," the Middle East Strategic Alliance (MESA) — that is supposed to “push back” against Iran. Against this backdrop, however, Iran does not seem worried about US efforts to create MESA. In fact, Washington’s new anti-Iran campaign in the Middle East and beyond is interpreted as falling within the “milking logic” of the Trump administration.