How does the international security environment influence whether and how military regimes democratise? This paper argues that for militaries in power, sustained external threats facilitate democratisation by credibly assuring the armed forces of continuing influence after leaving office. The credibility of this assurance stems from the military’s monopoly on the provision of national security and the reliance of all parties on the armed forces for the country’s defence. Militaries, confident of their continued influence after returning to the barracks, are more likely to cede power to democratisers when facing prolonged threats from abroad. Utilising a comparative case study of ruling militaries in Burma and South Korea, this paper tests four implications of the theory for how crises over democracy unfold between governing militaries and the opposition in contrasting security environments. It finds support for each of the implications.