Originally Posted by Amled

Case in point being Hannibal after the Battle of Cannae, where he never had the numbers nor the siege train to march on Rome, nor any of the heavily fortified Roman cities.
Also as Quintus Fabius Maximus showed when faced with Hannibal. In that Rome had an uncanny skill of fielding new legions; especially when fighting in the home provinces, in no small part due to their shorter lines of communications.
Whereas, that unless the Mongols had access to Chinese siege expertise, then they too might win battles and loose the war, due to their extended lines of communications.