How did Roman military law develop from the authority of magistrates who commanded troops?

In Roman society, military law developed directly from the imperium—the supreme authority held by magistrates, such as consuls, in their capacity as military commanders. Just as certain sectors of Roman civil law evolved from this magisterial power, military law was derived from the commander's prerogative to enforce discipline and ensure that their will was executed within the ranks.1

During the Republic, this system of military justice was often informal and heavy-handed, with practices varying significantly depending on the individual commander. Because the centuriate assembly was military in nature, it was responsible for electing the magistrates who exercised this power, linking the command structure to the Roman political system.12

Over time, this authority became more formalized. By the 1st century CE, military justice remained somewhat rough, but it eventually underwent significant codification 400 years later in the Digest and Codex of the emperor Justinian. This transition reflects the broader Roman legal evolution from magisterial discretion toward a more structured, written legal framework.13