This paper locates and traces the development of a norm against military interference in political affairs. As far back as 49 B.C., the Roman Senate forbade the Governor of Gaul, Julius Caesar, from commanding his troops to cross the Rubicon, a stream that separated Gaul from Rome. Entering Rome would require Caesar to cede authority to the Roman Senate. Thus when Caesar brought his troops across the Rubicon he violated the Senate’s control over the Empire’s military officials, sparking a civil war. Using this example as a starting point, this essay draws on theories of the state and colonialism to trace how the norm of civilian control over military officials developed through the period of European state development and Europe’s colonial state administrations. The study uses case studies of the former Syrian and Iraqi Mandates to draw out the global spread of the norm. Aware that crossing the Rubicon was a turning point in Rome’s politico-military relations, Caesar famously stated, “The die is cast.” This essay finds evidence that coup perpetrators and their opponents in Europe and its former colonial territories expressed similar sentiments about the military’s entrance into political affairs.