This chapter surveys military and operational aspects of the main guerrilla and counter-insurgency campaigns across Axis-occupied Europe and Asia. It emphasizes historical precedents, concepts of insurgency and counterinsurgency, and how far expectations for such warfare were met. Special Operations Executive initially sought to optimize the use of resistance through the detonator strategy: building, supplying and training vast numbers of fifth columnists. Between summer 1943 and spring 1944, waning Axis military fortunes not only benefited guerrilla movements. In the Soviet Union, the military shift against Germany following the end of the Battle of Stalingrad in February burgeoned in the months after the July Battle of Kursk. During that battle, moreover, guerrillas carried out extensive sabotage against German supplies and reinforcements, the first significant example of guerrillas working as an adjunct to conventional operations during the Second World War.