Research question - Which conditions are unique, regardless of time and space, for; supplying units in the defense of one's own territory, and supplying units in an invasion of another nation's territory? Purpose – This study aims to increase the understanding of the differences in logistical conditions for strategic supply, and to contribute to the relatively poorly explored research field of military logistics.  Design/methodology/approach – To answer the research question, the author conducted a structured, focused comparative study on three conflicts; The Finnish War, 1808-1809, Operation Barbarossa, 1941, Operation Iraqi Freedom, 2003. Findings – None of the defending sides were properly prepared. The defenders lost resources to the attackers. The defender controlled the territory before the conflict, and had the opportunity to affect and prepare it. All attackers could choose time and theater of war. All attackers’ units were cut off from their supply source due to weather or climate situation. The attackers had the opportunity to obtain resources from the defenders. The distance between the attackers’ units and their rear supply source increased as the offensive continues.  Originality/value – This is the first study that attempts to identify general differences between supplying the defending and the attacking side during an invasion.