Existing scholarship ignores relational interdependencies when attempting to understand the behaviour of non-state armed groups during civil war. This paper investigates the in- terconnected web of alliances and rivalries in the Yemen Civil War to answer the following question: why do armed groups fight each other? We employ a network approach to investigate determinants of intergroup violence. This emphasises the role of identity, arguing that operating in salient cleavages necessitates that groups align or distinguish themselves from each other. We further argue that informal cooperation incentivises violence the longer the war continues. These arguments are tested using pooled Exponential Random Graph Models to account for endogenous structures over time. Results indicate that shared identity is a significant driver of hostilities, moderating cooperation and amplifying the effects of group attributes. Robustness checks and simulations demonstrate that network models more accurately capture the underlying mechanisms to predict fighting in this case.