Domestic military operations in democracies had historically been rare, reserved for extreme emergencies. However, the proliferation of unconventional threats, notably climate change, biosecurity hazards, and terrorism, has significantly increased the frequency, complexity, and duration of domestic deployments. This book provides a global comparative study of domestic military operations, with a specific focus on the COVID-19 pandemic. The diverse 26-country contributions investigate how varying national contexts, governmental systems, and military structures influenced these deployments, as well as the impact on civil-military relations. While the pandemic created similar demands, states responded differently, allowing for the identification of broader patterns. The research examines the legal frameworks for domestic operations, the specific roles militaries played during COVID-19 (e.g., medical support, logistics, security), and implications for civil-military relations. The case studies show that due to their readiness and organizational capacity, militaries across the globe are increasingly called on domestically as a “force of last resort.” However, extensive deployments and so-called “mission creep” are straining resources for force generation and sustainment, affecting operational readiness, especially for core combat missions. This trend challenges traditional conceptions of civil-military relations theories, raising concerns about blurred roles and professionalism. The volume draws on principal-agent theory for a more nuanced understanding of civil-military dynamics during the pandemic: political executives used the military as an agent, with heightened risk for "agency slack" or "securitization," depending on civilian oversight. The volume concludes that the armed forces need to be versatile, agile, and adaptable as "guardians of the nation" in response to the evolving security environment.