This paper argues that security cooperation among neighbouring countries in the Global South is often hampered by domestic instability and fragmented territorial control resulting from state failures. Geographical proximity, characterised by porous borders and high levels of cross-border human mobility, directly impacts the security of neighbouring states. This creates a dilemma for security cooperation when one state lacks the capacity for effective governance. Empirically, the paper examines the evolution of Thailand’s security relations with Myanmar over recent decades, highlighting the profound impact of Myanmar’s political instability on Thailand. It analyses how the 2021 military coup and the subsequent collapse of Myanmar’s domestic political order have shaped Thailand’s securitisation of non-traditional security threats. By focusing on issues such as irregular migration, public health issues, drug trafficking, and transboundary pollution, the paper explores how these challenges have been securitised in Thailand and how they have complicated security cooperation between the two countries. The paper contends that the limited territorial control and legitimacy of Myanmar’s military government have significantly hindered Thailand’s ability to address its security concerns effectively. It further calls for security cooperation in the Global South beyond the conventional state-to-state level.