
Map of all the territories once occupied by the Roman Empire, along with locations of limes and borders (public domain image)
(12) DEFENSIVE IMPERIALISM
“Now look at what you made me do!”.
Alternatively, I needed my empire to defend myself.
As ridiculous as it may seem, empires have asserted that their imperialism was defensive or reactive, and historian have even agreed with or asserted it for them.
“Defensive imperialism is a security-motivated theory of empire-building that views territorial expansion as a reactive response to external threats rather than planned, aggressive conquest…states often conquered reluctantly out of fear, self-protection, or broken treaties”.
The most famous example of defensive imperialism is the one that may also seem most surprising, the Roman Empire. Indeed, the theory of defensive imperialism originated “in Roman studies in the late 19th century”, albeit it persisted after that and was “closest to the argument favored by the Romans themselves”.
“The theory draws its strength from the apparent inconsistency of Roman foreign policy, the slowness with which their rulers acted (or reacted), and their reluctance to annex. In this pattern economic and ideological factors appear secondary at most.”
Perhaps not coincidentally given the theory was contemporary with the British Empire approaching its peak, defensive imperialism was also asserted for the British Empire – “in the late Victorian and Edwardian eras, figures like James Bryce compared Roman expansion to the British Empire, arguing that external dangers and frontier instability forced further conquest”.
Admittedly, claims of defensive imperialism are the exception rather than the rule, particularly for pre-modern empires that tended to cheerfully revel in conquest for its own sake – what is best in life is “to crush your enemies, see them driven before you, and hear the lamentations in their women”. That quote from the Conan film is famously adapted from one attributed to Genghis Khan in a historical biography of Genghis Khan (Charles Lamb’s “Genghis Khan: The Conqueror of All Men”), which notably also included “to take their horses and goods”. While the latter was probably apocryphal, it certainly reflected the ethos of the Mongol Empire.
Even so, claims of defensive imperialism are significant. After all, two of the top three empires in my Top 10 Empires have asserted it for themselves or had historians assert it for them, the Roman and British Empires – and another entry, the Macedonian Empire, might arguably be added to them, at least in so far as Alexander the Great claimed his invasion of Persia to be a “retaliatory defensive act of justice” (for the Greek-Persian Wars and Persia’s control of or interference with Greek cities).
Claims of defensive imperialism haven’t exactly gone out of fashion either – “scholars point out that ancient and modern powers frequently weaponize defensive language and just-war theory to legitimize pre-emptive strikes and territorial control”.


