Under the Westphalian system of sovereign states, the military was usually seen as an important instrument of diplomacy. The states used the military as an effective instrument of diplomacy through the threat of the use of force or, in the case of a militarily weaker state, through the ability to withstand military attack or by engaging itself in attrition. Those days, the states had little choice but to defend themselves against actual threats to use militaries. Under international relations, military forces are deployed for legitimate self-defense. It remained central to the competition between great powers, which defined the global order during the Cold War era. However, in the post–Cold War era, due to globalization and technological innovations, threats came from various sources, including cyberspace, cross-border insurgency, and terrorism. The militaries, in their traditional form, could not deal with them. They had to coordinate with civilian and political power groups at subnational, national, and supranational levels, which severely affected international relations. Post-modern militaries were required to fight for human security and promote justice, peace, and democracy. This chapter explores the current and anticipated “challenging shifts” and “ambiguities in local and international contexts” surrounding the myriad situations involving the military in the future.