How Rome Defended Her Borders: If You Want Peace, Prepare for War

The Roman Empire, like any other Empire, had to secure its border. There were two types of threats that the Empire had to deal with: On the one hand, foreign powers seeking to invade and conquer parts of the Imperium Romanorum itself, on the other hand tribal groups that pillaged border areas any change they got. People passing the borders for trade, employment or education was common and there was otherwise little welfare that could attract unwanted migration. As we will see, it was much more difficult to defend borders with barbarian tribes than borders with civilized realms, even though the latter could easily levy bigger and more combat-worthy armies.

The specific structures of border protection varied through the more than thousand years that Rome, at least the Western part, existed. However, the general idea was always the same. But first, we have to distinguish between two types of borders: Borders with civilized realms and borders with the wilderness, inhabited by savage tribes. The former were mostly the borders to the east, that were different Empires and Kingdoms, some of which had been vassalized by Rome. Most of the Empire’s borders to the north and south were open wilderness, like the deep, dark forests of Germania or the hot Sahara and Arabian deserts. The Atlantic Ocean was the final frontier to the west.

I. Borders with Civilized Realms

Borders with civilized realms were pretty straightforward: The border was somewhat specified according to agreements. Garrisons situated at border crossings collected tariffs from travelling merchants. A border defense was necessary if the realm across the border posed a threat to Rome. The general strategy behind Roman borders was twofold: Soldiers were stationed at the border and garrisons in the hinterland, with cities being fortified. Those local soldiers often lived a somewhat normal life, running small businesses or tending to fields, having female companions and children. In case of an attack, these local soldiers were expected to send message to Rome and delay the enemy’s advance while withdrawing into the fortified cities. The enemy usually would lay siege at these cities. At this point, the defenders could only hope that the reinforcements dispatched by Rome arrive in time. These reinforcements, unlike the local troops, consisted of highly trained veterans that had seen many battles as members of units that were dispatched throughout the whole Imperium.

II. Borders with the Wilderness

Defending a border from a civilized enemy was an often brutal and expensive task, but comparatively easy to execute: You would know where your enemy is and you just had to destroy him, the latter usually being the bigger challenge. The opposite was the case with the borders to wilderness: There, the issue at hand were not huge armies but small warbands who wanted to plunder the rich Roman cities and farms, maybe capture some slaves. Killing these warbands, who might have a handful of fighters up to a few hundred at really big raids, was an comparable easy task for the Roman Army: These raiders were neither trained as good as a Roman soldier, nor was any of these thieves keen to die in battle if running was an option. However, the issue was locating these highly mobile warbands who tried to hide their intrusion and sneak away with their spoils. Think of borders like the border between the US and Mexico, where many can slip through despite the technology used nowadays.

That being said, how did such a border to wilderness look like? First at all, there usually was no defined border. If available, natural borders like rivers were chosen, because they made it harder for warbands to cross them undiscovered. This was the case with the River Rhine and Danube bordering to Germania Magna. Sometimes, border walls were build with the same purpose. Think of Hadrian’s wall in Britannia. In any case, like a mesh, garrisons and outposts were spread over the whole border area, which, in theory, were able to surveil the whole area and detect any intrusion. If a warband was detected, the local soldiers would attack and destroy it. Other locals would reinforce them, since each post ideally was within range of sight to other posts. There were border crossings that collected tariffs, but the actual border area had a depth of dozens of kilometers, starting with the first outpost deep in the wilderness until big cities, the tariff collection point somewhere in between.

Sometimes, tribes would petition Rome to allow them to settle within the Empire’s borders, because living under Roman rule meant protection and peace. Rome would sometimes allow this, but disarmed the tribe’s members and spread them through a larger area, forcing the tribal members to integrate with the locals. This strategy was important to prevent any uprising that would be fueled by shared ethnicity and culture.

III. Rome’s Collapse

Rome collapsed when the Empire was not able to defend its borders anymore. The reasons for this are far too complex for this video, but in short, Rome was not able to pay for the manpower needed to run this expensive border security. Furthermore, whole stretches were left exposed when the local troops were mobilized to reinforce other borders or fight in civil wars. This allowed warbands to raid the adjacent provinces unopposed and to venture deep into the Empire, destroying Rome’s economic backbone. In the end, whole tribes crossed the barely defended borders and started to divide the bankrupt Empire; others who were already in the Empire as mercenaries started to claim territory after Rome couldn’t pay them anymore.