As Luttwak notes, modern historians and military theorists have a tendency to sneer at linear defense lines.1 In fact, some historians of ancient Rome actually blame the decline and eventual collapse of the empire on all the “wasted” energy spent building frontier fortifications. The argument against such “cordon” defenses is that for a given quantity of military potential, spreading it out equally along a perimeter and trying to guard every spot equally dilutes your strength. This makes it easy for an attacker (who picks the time and location of the battle) to concentrate his forces, create a local advantage, and break through.
The thing is, approximately none of this logic applied in the Roman situation. First of all, as we’ve already noted, a huge fraction of the threats the Romans faced were “low-intensity”: border skirmishes, slave raids, pirates and brigands, that sort of thing. Static fortifications, walls and towers, are often more than sufficient for dealing with these problems. Paradoxically, that actually increases the mobility and responsiveness of the main forces. If they aren’t constantly running back and forth along the border dealing with bandits, that means they can respond with short notice to “high-intensity” threats (like major invasions and rebellions) that pop up, and are probably better rested and better provisioned when the emergency arrives. So, far from diluting their strength, a lightly-manned series of linear fortifications actually enabled the Romans to concentrate it.
Secondly, those linear fortifications can also be very useful when that major invasion shows up, even if they are overrun. A defense system doesn’t have to be impenetrable in order to still be very, very useful. One thing it can do is buy time, either for the main army to arrive or for some other strategic purpose. The defenses can also act to channel opposing forces into particular well-scouted avenues of attack, or change the calculus of which invasion routes are more and less appealing. Finally, in the process of setting up those defenses, you probably got to know the terrain extremely well, such that when the battle comes you have a tactical advantage.
[…]
The third, and perhaps most important, reason why the Roman frontier fortifications were actually very smart is that they were carefully designed to double as a springboard for invasions into enemy territory. Luttwak coins the term “preclusive defense” to describe this approach. The basic idea is that an army can take bigger risks — pursue a retreating foe, seize a strategic opportunity that might be an ambush, etc. — if it knows that there are strong, prepared defensive lines that it can retreat to nearby. Roman armies were constantly taking advantage of this, and moreover taking advantage of the fact that the system of border fortifications was also a system of roads, supply lines, food and equipment storage depots, and so on. The limes were not a wall that the Romans huddled behind, they were a weapon pointed outwards, magnifying the power that the legions could project, helping them to do more with less.
John Psmith, “REVIEW: The Grand Strategy of the Roman Empire by Edward Luttwak”, Mr. and Mrs. Psmith’s Bookshelf, 2023-11-13.
- I, an ignoramus, assumed this was all downstream of the Maginot line’s bad reputation, but Luttwak says it’s actually the fault of Clausewitz.
February 18, 2026
QotD: Defending the borders of the Roman Empire
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