For the week I read this book, I was incredibly worried that a friend could bump into me on the train, catch me reading, and then see the pompous title “The Grand Strategy of The Roman Empire.” Nevertheless, my respect for Edward Luttwak is so great that I read on and learned a tremendous amount from him. For the first time, I understood how comparatively disorganized German tribes could implode the world’s most powerful Empire with a merely symbolic victory.
Luttwak distinguishes three periods in the Western Empire. The first was a hegemonic system, where Rome exercised indirect rule through formally independent local actors. To keep eastern kings and German clans tame, heavily armed Roman legions were dispersed throughout the Empire’s interior. The system was cheap for Rome but unstable for its subjects. Then came the walls. Hadrian’s Wall dividing civilization from Scotland was the most famous of a series of barriers all over Central Europe, the Near East, and Africa. Legions were distributed to these perimeter defenses, and Rome began to directly rule and culturally assimilate the Mediterranean world. This system offered greater security to millions but was ultimately precariously dependent on an overwhelming military superiority. Eventually, that superiority waned and the Empire entered the last system, a deformed version of the second. Stretched by the demands for security outstripping its supply, Rome adopted a policy of defense in depth. This allowed medium-term incursions and occupations past the Empire’s border defenses. While it proved an economical way of fighting, a strong sense of Roman identity decayed, unto the Empire’s implosion.
The beginning of Empire was the end of the Republic. Rather than conceiving of itself as a unified political entity encircling the Mediterranean, the early empire was by, of, and for Romans. The rest of their territory were governed by dependents of minutely differentiated status, often given the title Friend of the Roman people. To keep everyone in check, the Legions were deployed to the Empire’s urban centers and agricultural heartlands. The ordinary problems of internal order or external raiding were handled by auxiliaries, the light infantry or cavalry raised by the local rulers. By contrast, the Legions were units of heavy infantry who spent more of their time as builders than warriors. They were constantly on the move (albeit at glacial pace encumbered by armor and ballista) erecting make-shift camps of tents and low walls everywhere they went. Legions were staffed with civil engineers building road and aqueduct networks. Budding out from settled regions, they would clear Linares, roads perpendicular to the frontier allowing forces to move outwards and face external threats. What was the strategy? According to Luttwak, it was a strategy of escalation dominance. Heavy infantry was suited to large static battles such as taking and holding towns and fortresses, while disfavored in situations of maneuver, pursuit, or patrol. Romans outsourced low-intensity conflicts to their local constables while holding the trump card in their ability to wreck any rebells that stepped out of line. The conquest of the Jewish fortress Masada illustrates the idea. Romans were willing the devout multiple legions over years to liquidate the last of these defeated rebels. A siege would have checked any threat from these remnants. Instead, over those years, Romans performed miracles of civil engineering gathering mountain-scaling ramps to crack the fortress. Legions existed to punish these internal political deputies.
Under the second system, Rome was transformed from a Hegemonic Empire into a Territorial one. Rule was direct from the center rather than mediated through local kings, and the Legions were sent to guard the borders. Demonstrative of this change, the word “Linares” used to refer to access routes into to border region, but it came to mean the walls and roads running parallel to a defined border. Hadrian’s Wall famously divided what-would-become civilized England from savage soon-to-be Scotland. But, the Fossatum Africae stretched ~750 kilometers to protect the north African coast from Saharan raiders. Meanwhile, large patches of wall divided Roman from German Central Europe. Unlike Game of Thrones, the point of the walls was not to erect an impregnable defense. The walls prevented the entry and more effectively the exit of raiding barbarians on horse. Instead of sitting on the walls like armies clustered on the Maginot Line, Roman Legions would usually fight on the far side of the wall intercepting concentrated enemy hosts. In Europe, the boundaries of the Romance languages today closely track those ancient ~10-15 foot barriers. Ever wonder why Romanian is a Romance language? Well, Romania was covered by an outshooting peninsula of wall, while its east-west neighbors were not. The walls were military defenses, but also began to forge a sense of Roman identity that newly extended beyond Italy. Victorious soldiers were often rewarded with land on these newly conquered frontiers. They brought their patriotism, language, and respect for authority with them. Importantly, Rome chose to wall off lands that were receptivity to their style of agriculture, and so where wine production was possible Roman culture followed. Rather than just extracting wealth from potentially unstable and squabbling local kings, the Empire gave order, legality, and infrastruture the to the chunk of globe within their grasp.
Then, three new security challenges would leave a perimeter defense unsustainable and necessitate change. First, the Imperial succession mechanism descended into nearly uninterrupted violence. In the early Empire, the prestige of Julius and Augustus restricted the potential successors to a relatively narrow radius of kin. While these early contenders were often violent amongst themselves, by the late Empire, the throne was up for grabs to the military man who could take it. Expensive civil wars replaced Pretorian coups. Recognizing the massive waste of military power, Diocletian single-handedly orchestrated a constitutional revolution. Instead of a single emperor, power would be divided into four: an emperor in the East and West respectively each with an appointed successor temporarily acting as their deputy. The structure would seek to vest the four most powerful generals into preserving the system rather than fighting to overthrow it. Though the reform stuck, the attraction of civil war only ebbed not ended. Second, there was the return of Rome's only peer power since Carthage: Persia. In 224 AD, the Sassanid Empire, a centralized urban civilization, replaced the Parthians, a steppe-based horse-lord dominion. The Romans had fought intermittent wars with the Parthians often with the limited objective of keeping Armenia neutral. The Sassanid threat required the permanent deployment of multiple legions to the east. Lastly, and ultimately fatally, large tribes German organized. They began raiding the coast of France from the sea, and powerful German kings reappeared after a century-and-a-half absence.
Newly stretched, Rome was slowly forced into a strategy of Defense in Depth. Faced with an invasion of your territory, there are three options. A Perimeter Defense where you spread your force to staff fortifications across your entire border. An Elastic Defense dictates that you abandon all entrenched defenses and fight the enemy on equal footing as a concentrated, mobile army. Of course, this allows the enemy to fight the war on your territory, provisioning themselves by expropriating your subjects. Defense in Depth tries to capture the advantages of both. Most of your forces retreat from the border to concentrate into a mobile army, but you leave heavily fortified castles on the frontier that can withstand assault and bare siege. These fortresses tie down the enemy army forces, launch raids of enemy supply lines, and can provision any counter-offensives that reach them. Of course, this type of siege warfare begins to resemble something more medieval than ancient, and Luttwak documents how many technologies associated with Medieval castles and cavalry began in this late Roman period.
Why did Rome collapse? Gibbons answers very roughly that cultural decadence sapped the Martial energy and competence of Rome. The fact that Legionaries carried fewer javelins and types of swords in the late Empire is often cited as a telling degradation. Luttwak argues that many such changes were rational. For example, it made sense to equip the Legions differently when they were acting as heavy infantry enforcers than when they were the Empire’s main source of manpower. Instead, in Luttwak’s telling, the Empire collapsed from its internal contradictions. With an important trigger.
In itself, the sack of Rome was largely symbolic, the Germans responsible for it could not hold that portion of Italy. The Eastern Empire took back Rome and continued ruling the most prosperous and important parts of the Mediterranean from Constantinople (while holding onto the old capital of Rome with its immense symbolism). However, the Empire in the West was irreparably unwound. Why? A sense of Roman identity has slowly degraded in the Western Empire, as a strategy of Defense in Depth unassimilated Germans to invade and settle within the Roman walls. What had been a demarcated zone of peace and trade became looser. Local strongmen began providing security again, and the prestige of a central Empire slowly eroded unto its death.