[muawiyah]

The Mongol "build up" occurred over several centuries prior to Genghis, but his conquests were not a "volkswandering" sort of thing ~ more like an organized invasion by a major military power.

I think what the researchers focused on here were the internal Chinese disturbances ~ which took place even under Mongol control.

[hellbender]

Yes, and I think that “Mongol build-up over several centuries” coincides with the Medieval Warm Period. Global warming helps people in northern latitudes.