Suicide rates among veterans and military service members have reached unprecedented levels. This underscores the critical need to understand risk factors for suicide in these populations. Combat-related killing is reported by a substantial number of military personnel and veterans and is associated with negative psychiatric and functional outcomes. Killing has been identified as a potentially important and understudied risk factor for suicide in military personnel and veterans. This chapter proposes the interpersonal-psychological theory of suicide (IPTS; Joiner, 2005) as a theoretical model for understanding the relationship between combat-related killing and suicide. The IPTS proposes that thwarted belongingness and perceived burdensomeness cause suicidal desire, whereas prior experiences with pain, provocation, and death result in habituation to the fear and pain associated with suicide, thus influencing one’s capability for suicide. The chapter concludes with a discussion of clinical implications and recommendations for future research.