moral of this mathematical digression is that, on flat plains, with warriors using projectile weapons, any numerical superiority that an army can achieve over its enemy is magnified out of all proportion. In other words, Lanchester's Square Law yields an enormous return to social scale. If the opposing forces use a mix of ranged and shock weapons, numerical superiority will still be amplified, although not as much as with purely projectile weapons. So there is an intense selection pressure for cultural groups living in flat terrain to scale up, and a very high price to pay by those that fail to do so {recall where the first states emerged}. In the mountains the selection pressure for larger societies is reduced considerably. Wars
( Peter Turchin )
[ Ultrasociety: How 10,000 Years ]
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