This line of thinking may not be bullet proof but interesting nonetheless:
The Romans they were strong and expanded by conquering ever more territory. Soon they had an empire. They made infrastructure so that they could move faster, more easily, between different parts of the empire. So what happens next? Oh, suddenly mr Roman fucks a Spanish lady, oh, and a French one too. Oh genetic distance increases, ethnocentrism decreases. Oh, the empire suddenly for no reason at all gets weaker. Barbarians with strong/close genetic bonds easily invades and destroys the empire. Experts shocked.
I guess the TLDR version of this is:
pure blood men creates traveling traveling creates mixed blood men mixed blood men are conquered by pure blood men
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