And I'd hardly say the Mongols failed just because they couldn't hold to western Europe.
Charlemagne passed the entire empire to Louis the Pious, and Louis the Pious had three heirs that split the empire. See https://en.wikipedia.org/wiki/Treaty_of_Verdun
The direct heir actually didn't split. It was only later the families split.
> And I'd hardly say the Mongols failed just because they couldn't hold to western Europe.
I mean failed to hold Europe.
Charlemagne passed the entire empire to Louis the Pious, and Louis the Pious had three heirs that split the empire. See https://en.wikipedia.org/wiki/Treaty_of_Verdun