Salvete omnes,
Reading Susan P. Mattern's "Rome and the Enemy", I was surprised at reading that "It has often been remarked that under the Principate, Rome managed to secure, defend and even expand is empire with a relatively small military force". (p. 82). The author argues that "This small, fixed size of the army...was a key factor in Roman strategy" (p. 83).
She estimates the size of the Roman forces at about 400.000 men (p. 83). Not exactly a small army, I'd say, if we compare this to the relative military strength of modern nations.
It is generally accepted that, under the Principate, the Roman Empire had about 60 to 80 million inhabitants. If we take 70 million as a good estimate for our calculations, this would mean that app. 1 out of 175 Romans were active in military service.
Now, let us compare these figures with those of a few significant modern military powers.
The United States have an active military force of 471.500, out of a population of 295.734.134; that is 1 American out of 627 serving in the army.
The United Kingdom has an active military force of 113.900, out of a population of 60.441.457; that is 1 out of 531 citizens.
Russia has an active military force of 350.000, out of a population of 143.420.309; that is out out of 410 Russians.
If we take the average of these figures, I arrive at the result that the relative military strength of the Roman Empire was three times that of the modern nations mentioned here.
Two comments should be made upon this result, one relativizing the strength of the Roman army, the other making it look even more impressive.
The first comment is that a modern army, due to its uncomparable technological equipment, needs far less manpower to be effective; weaker numbers thus do not mean less performative armies.
A second consideration is that modern nations can command the resources of strongly developed industrial economies (with a much more extensive and weighty taxation system), much more productive and sizeable than Rome's relatively primitive, largely agrarian economy; this means that for Rome, the cost of maintaining such a large army was an uncomparably greater challenge.
Any comments about or corrections on my figures and conclusions ?
Valete,
Atticus