How does interstate rivals’ intervention in a third-party civil war impact conflict duration and outcome in that country? More specifically, (1) how do interstate rivals engage in third party civil conflicts and (2) how do conflict parties attract, utilize, and sustain the external support they receive from their supporters? I answer these questions by comparing Saudi Arabia and Iran’s intervention in two distinct conflicts in Yemen. This dissertation applies a qualitative case study method and selects two within-case methods: structured-focused comparison and process tracing. In the structured-focused comparison, I compare the support Saudi Arabia and Iran has provided to the Government of Yemen (GoY) and the Houthis respectively, differentiating between military and non-military types of support. I compare these during the Saada wars (2004-2010) and in the current internationalized civil war (2014-2018). To ensure the validity of my causal inferences I triangulate data from three sources: the development of a novel mediation and ceasefire dataset1 (1), semi-structured in-depth elite-interviews2 (2), and document reviews (3). This dissertation develops a mechanism-focused analytical framework that integrates both rivalry and civil war dynamics to explain civil war duration and outcome. I build on five distinct literatures (strategic rivalry, civil war studies, third-party intervention in civil wars, mediation, and rebel governance) and complement them with the literature on Middle East Area Studies. In the analytical framework first, I look at the inter-state dimension and propose that rivals’ initial decision to intervene and their subsequent decisions to remain engaged in third-party civil wars are two distinct processes. Rivals seek to inflict costs on their counterparts, but at the same time they seek to avoid direct confrontation. Their cognitive rigidities lock them in their own conflicts and give rise to the mechanism of conflict integration in third-party conflicts. Besides interveners, I also take into account domestic dynamics and examine civil war conflict parties’ capacity to impact rivals by keeping them engaged in their conflict through the mechanism of rivalry instrumentalization. The external and internal perspectives are reinforcing each other and create networked interdependencies. With this two-dimensional logic, I move beyond the conventional framework of proxy wars.