[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.

[muawiyah]

In the movie "Mongol" one of the things they did right was to depict the Mongols of Temudgen's childhood as rather impoverished rural rustics chasing sheep around. By the time he'd become the Khan of all the tribes they were as well outfitted and trained as the Chinese.

I suspect the lack of competition from Siberia to the East (most of the earlier militarily advanced Yakuts/Sakha having already gone to Korea, and then Japan in the 500/700 AD period) enabled the Mongols to operate in a vacuum and thereby escape Chinese notice until it was all too late.