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.