This paper will discuss the Resistance Operating Concept and how nations should prepare to resist a potential enemy before an invasion takes place. Oriented towards the self-defense of small countries by a resistance or partisan force, it describes past examples of resistance groups in Europe. Specifically, by discussing the long-term survival of resilient organizations, its focus will be on the basic factors crucial for an underground resistance, including security, organization, and training. It also considers the need for a practitioner-oriented manual that can be disseminated at the widest levels to guide and enable future resistance operations.