The authors examine the military implications of intrastate proxy wars: civil wars in which at least one local warring party receives material support from an external state. The research was conducted using a review of existing literature and case studies of four particularly relevant instances of proxy warfare, including the First and Second Indochina Wars, the 2014-early 2022 Donbas War, and the Houthi Rebellion. At the strategic level, the increased lethality of violent nonstate actors (VNSAs) complicates traditional models for responding to insurgencies and other forms of irregular warfare, while the risk of escalation forecloses potential options for responding to these challenges. At the operational level, state-supported VNSAs' combination of lethality and greater capacity for dispersion can impose multiple dilemmas on forces like those of the United States. These strategic and operational challenges have implications for U.S. Army doctrine, education and leader development, training, and potentially personnel and organization.