This article examines how the Sudan People's Liberation Movement/Army (SPLM/A) developed distinctive governance repertoires during its four-decade insurgency (1983-2005) and how these wartime institutional practices fundamentally shaped the architecture of the post-independence South Sudanese state. Drawing on rebel governance theory, historical institutionalism, and Migdal's state-in-society framework, this study argues that the governance practices developed by armed movements during insurgency are constitutive of - not merely antecedent to - post-conflict state institutions. Through process tracing of SPLM administrative, taxation, and justice practices from the New Sudan period through independence, this research identifies the mechanisms through which rebel organisational culture - including factional competition, personalised command structures, and predatory extraction - was translated into post-independence governance pathologies. The findings demonstrate that the SPLM's wartime governance legacy has produced a hybrid state formation trajectory characterised by the institutionalisation of armed movement practices, with profound implications for post-conflict statebuilding theory and practice in Africa.