Recent theoretical and experimental investigations of altruistic behavior in intergroup conflict in humans frequently make use of the assumption that warfare can be modeled as a symmetrical n-person prisoner's dilemma, abstracting away the strategic differences between attack and defense. In contrast, some empirical studies on intergroup conflict in hunter-gatherer societies and chimpanzees indicate that fitness relevant risks and potential benefits of attacks and defenses might have differed substantially under ancestral conditions. Drawing on these studies, it is hypothesized that the success of defenses was much more important for individual and kin survival and that a disposition to act altruistically during intergroup conflict is thus more likely to evolve for the strategic situation of defense. It is then investigated empirically if such asymmetries in the occurrence of altruistic behavior during intergroup conflict can be found. Analyzing detailed historical case data from 20th century wars, this study finds that altruistic behavior towards members of the in-group indeed seems to occur more frequently when soldiers are defending themselves and their comrades against enemy attacks. It is proposed that this asymmetry reflects adaptive behavioral responses to the materially different strategic character of attacks and defenses under ancestral conditions. If true, this would call for a refinement of theories of the evolutionary interaction of intergroup conflict and altruism.