This chapter explores denationalization, focusing on Indonesian foreign terrorist fighters (FTFs). Post-9/11 and during the Arab Spring, Western democracies tightened border control to combat terrorism, enabling the stripping of citizenship from involved individuals. Denationalization, via law or public-authority decisions, emerged as a contentious counter-terrorism tool. Indonesia, as a Muslim-majority Southeast Asian nation, partially embraced denationalization, refusing to repatriate Indonesian FTFs. This aligns with global security concerns but raises statelessness questions. The chapter examines denationalization’s legal framework, international obligations, and the blurred line between de jure and de facto statelessness. Critics argue that disproportionately applying denationalization to Muslims undermines human rights, inter-state cooperation and international justice. Refusal to repatriate Indonesian FTFs raises concerns about long-term consequences and due process. Understanding denationalization nuances is vital, considering its impact on the citizenship rights of individuals involved in terrorism.