The entire Mediterranean Sea was within Rome’s borders during the third century. It controlled much of what is now Europe, Greece, Asia Minor, Syria, Egypt, all of Spain and Portugal, and Africa from the coast to the edge of the deserts.
Conquering all these regions involved armies concentrated at specific places at specific times. But controlling, administering, and protecting all of this vast and sprawling area simultaneously was a significant challenge. Part of the solution was an excellent, extensive road system built and maintained by Roman engineers. Men, equipment and supplies could be moved anywhere within the empire efficiently.
Today we are accustomed to nearly-instant worldwide communications. If there is an earthquake in Turkey, a volcano eruption in Indonesia, or an outbreak of disease in central Africa, we will know about it within hours. In Roman times information moved by ship, on horseback, or by foot. News of similar disasters could take weeks to reach Rome and relief could take a month or more depending what was needed and where it needed to go.
More pressing issues such as invasions by hostile tribes to the north, or Persians to the east required much more rapid responses. Consequently, Rome divided the empire into a number of provinces, each managed by provincial governors, who commanded military forces in his province. The number of legions assigned to a province varied over time but was based on the perceived threats in that region. Governors were assigned by Rome and were replaced periodically after serving 3 to 5 years. Provinces needed a competent administrator and general, but sometimes the provincial governor was persuaded to claim the title of emperor himself. This usually happened by acclamation from the legions, after one or more impressive victories over enemy forces.
Following a provincial revolt, the general usually took part of the forces under his command and marched toward Rome to challenge the existing emperor. The remaining legionaries in a province were usually insufficient to resist barbarian invasions. After overwhelming local defenders, invading tribes were free to roam the province sacking cities and villages, kidnapping local inhabitants, later sold into slavery, or occasionally ransomed, and killing many of the remaining citizens. Naturally this alienated the surviving populations.
A response from the emperor, or forces loyal to him, was somewhat similar. They needed to assemble an army sufficient to resist the invaders. A confrontation might occur between two Roman armies. If a battle was fought, all casualties were Roman soldiers and depleted the remaining armies’ ability to protect their borders. However, if one of the armies was noticeable smaller than the other, soldiers from the smaller group sometimes killed their leader and claimed allegiance to the leader of the larger army.
This scenario played out frequently. Revolts and struggles for succession had been occurring since the time of Augustus. Doubtless this was at least part of the reason 14 emperors had died violently in the 42 years prior to Gallienus becoming emperor, all but one killed by other Romans.
