The Syrian conflict is a useful case study to explore Iranian and Saudi Arabian rivalry. 1 Both states took an active interest in the civil war from the beginning, each seeking an outcome that would benefit its wider regional ambitions. Iran, allied with the Ba'ath regime in Damascus since 1979, resolved early on to aid President Bashar al-Assad in the face of first demonstrations and then an armed insurgency. This initially consisted of an 'advisory mission' in early 2011 but, within a few years, had ballooned into vital economic and military support that allowed the embattled Assad to survive. Saudi Arabia, in contrast, was slower to oppose its rival's ally. Before the uprising ties between Damascus and Riyadh, never especially friendly, had warmed and King Abdullah was cautious to condemn a fellow autocrat for crushing protestors. However, by late 2011 Saudi Arabia had decisively turned on Assad, calling for his departure and urging sanctions. A year later it was sponsoring the opposition movement, sending weapons to armed rebels and urging its ally, the US, to intervene.However, after a decade of war, Assad has clung on to power. Though it has been costly, Iran is now deeply embedded in Syria, while Saudi Arabia has effectively given up on ousting Assad, ending support for the opposition. This chapter asks how we can explain this comparative strategic defeat for Riyadh and victory for Tehran. It draws on debates within international relations scholarship to ask whether either had a structural advantage going in the conflict, or whether the outcomes were more the result of the decision-making of the ruling elites. Can Iran's success in Syria be explained primarily by structural factors as systemic realists would argue, with a focus on the international and regional system, material capabilities and international alliances? Alternatively, were ideational tools, such as utilising Shia identity and anti-Western ideology the key to its success, as constructivists would emphasise? Or is Saudi incompetence the better explanation, placing more focus on the domestic factors limiting Riyadh's foreign policy effectiveness, in contrast to more domestic security from Tehran, as neoclassical realists might argue? This chapter will suggest that all played a role. While