This study focuses on the unique characteristics in integrating the historically overt Territorial Defence Forces (TDF) with clandestine underground resistance organisations as part of efforts by various countries to build national resistance capacities prior to a conflict. This paper provides the theoretical and historical underpinnings of the concept of TDF and underground integration, including observations from the 2022 Russian invasion of Ukraine. Case studies of the Polish Home Army’s integration into the Polish Underground State in WWII under primarily German occupation and subsequently the Polish Independent Underground until 1963 under Soviet occupation are used to better understand the unique aspects of TDF and underground integration. In both cases, the respective TDFs were operating against equally brutal but distinctly different occupiers. The case study analysis identifies and discusses three key lessons for integrating military and civilian capabilities in national resistance programs built prior to a conflict: 1) the criticality of civilian control, 2) ambiguity, protractedness, and the TDF, and 3) the scaling the TDF and underground. Finally, recommendations are offered to support the implementation of the lessons learned. While these lessons and recommendations are focused on TDF and underground resistance organisation integration, they also similarly apply to every ministry, department, and agency of nations developing similar capabilities and may enable the successful implementation of related efforts. No single ministry or department can effectively establish a viable national resistance organisation in a vacuum. This research also sets the conditions for further distinct analysis to increase the theoretical understanding of these concepts.