biffvernon wrote:World population was rather small in Roman times, so that can hardly account for their army.
That wasn't the point I was making. Due to slave labour (ie cheep energy) used on the latifunda, the Romans had enough spare population to maintain a large standing army. As the Empire spread and Romanised everywhere they went, they had even more "spare" people, hence the introduction of the Auxilia after the Marian Reforms. The fact they had such a huge (for the times) supply of spare bodies was why they conquered Europe, North Africa and Asia Minor by the latter half of the 1stC BC.
For example, under Augustus, there were 28 full legions, that's 143,360 men doing
nothing but soldiering for 25 years. Well, that and building roads, of course.
If anything, it helps to support your argument. I doubt that we can support a world population of 9 billion, but we can certainly manage to maintain pre-WW2 levels of population no sweat. Add Terra Preto into the equation and I'm pretty sure we can sustain 6 billion, but I think 9 billion might be pushing our luck, especially if it is decided to grow fuel for the infernal combustion engine rather than fuel for humans.