This article develops ethnic militarisation by reform as an analytical lens for understanding how security sector reform can entrench ethnic militarisation instead of producing a national force. Rather than treating the topic as a descriptive case note, the manuscript positions security sector reform as ethnic politics: armed forces composition, factional command, and the militarisation of ethnicity in post-conflict south sudan within wider debates on African political order, state formation, and institutional design. The paper is anchored in South Sudan, Rwanda, and the Democratic Republic of the Congo, and it uses the topic brief's theoretical architecture to ask three linked questions: how has the composition of south sudan's unified armed forces formally integrated through ddr and ssr processes continued to reflect and reinforce ethnic patronage networks rather than produce a national institution? What role do Dinka, Nuer, and Equatoria ethnic identity logics play in determining promotion, command appointment, and resource allocation within the SSPDF and how do these dynamics interact with factional loyalties within each community? Under what conditions does international SSR assistance inadvertently legitimise and resource ethnically structured armed forces, and what design changes would alter these incentives? Methodologically, the article translates the proposed design into a publication-ready strategy built around analysis of sspdf officer rosters and promotion records; interviews with ssr advisors (unmiss, eutf, bilateral); process tracing of ddr implementation 2006–2023; comparison with rwanda's post-genocide military integration and drc's fardc integration failures.. It argues that the central analytical payoff lies not only in better explanation of the South