[mrexroad]

Likewise, having a population of 1-2B also introduces the risk of scaling beyond the food production capabilities of the land. Obviously an overpopulated country can (and likely will) take from an underpopulated country, but modern weapons create a new dynamic if an invading force attempting to occupation farm land or capture grain stores is viewed as mortal threat.

[Aelinsaar]

That's exactly it, and it creates a situation in which relatively poor, very densely populated countries fight with each other for resources, rather than attempting to take from an underpopulated country armed to the teeth. I think we've seen a preview in the Middle East and North Africa, and gotten a hint of how Western nations are going to respond to the inevitable influx of refugees.