This chapter discusses three theoretical perspectives that offer potential explanations of military behavior. From a realist perspective, military conduct reflects the type of problems soldiers face on the ground. A constructivist account stresses the power of international, transnational, and/or national norms. Liberal theories point at the domestic structures of democracies as the main cause of intervention practices. While these explanations offer insights, they underdetermine military behavior in the field. Realism glosses over organizational biases. Moreover, the assumption of rational action that cuts across much of realist writing is inadequate because rationality is often unclear in situations such as counterinsurgency. Also, militaries employ similar capabilities differently. A constructivist perspective on (international, transnational, and national) norms has limited explanatory power because protection norms leave militaries significant wiggle room. Furthermore, international organizations, as important norm entrepreneurs, struggle to ensure coherence among military organizations participating in multinational missions. A liberal approach focusing on political decision-making has difficulty accounting for variation in military conduct and underestimates the autonomy of militaries on the operational and especially tactical levels. While all of these approaches offer some insights, they cannot properly capture how soldiers implement their missions. For this, an organizational perspective is needed.