ABSTRACT The escalation of militarized interstate disputes (MIDs) into interstate wars has been studied extensively with opportunity and willingness frameworks. In this article, we conceptualize and operationalize proto-insurgencies as a civil war equivalent to MIDs. Just as most MIDs do not escalate into interstate war, most proto-insurgencies never make the key tactical transitions needed to produce the onset of civil war. We separately examine proto-insurgency formation and proto-insurgency escalation to onset. We find that proto-insurgency formation is associated with political persecution, such as denial of access to justice, regime corruption, and mid-range levels of repression. Our theoretical focus is on how state repressive violence creates diffusion effects that push proto-insurgencies toward the tactical transitions that enable the escalation to civil war. Proto-insurgency escalation to civil war onset is driven by high levels of state repressive violence directed against proto-insurgency and its area of origin. This leads to the dispersal of proto-insurgents and the displacement of surrounding populations, inducing the spread of state repression to a larger number of administrative units. This creates the foundation for guerrilla warfare and establishes the conditions for rebels to offer sanctuary to displaced persons in return for active support, marking the onset of civil war.