The fact is that he was already winning that war, he had won the previous battles and like you said he was trailing a retreating enemy,, fact is that duke wellington (think it was the name of one general of the Seventh Coalition army if I recall right) decided to make a "last stand" in near waterloo, they fortified strategical locations and bottlenecked the french army between houses and high ground (again if I recall right it had rained in those days so you can imagine how well you can move with an armor in mud and high ground), still the battle was turning in favor of Napoleon (I think he was watching it but well you can't really say for sure lol) in fact one side of the Seventh Coalition army was starting to break, unluckly for Napoleon it happens that the Prussian army (again if I recall right) shows up exactly on that side (no ideas if they even knew what was happening before they arrived), and since those troops where "fresh" while Napoleon's had battled for a while the french army started to break, then well there was caos, Napoleon to make sure other troops didn't lose morale said that the army that had arrived was part of the french army, the Seventh Coalition army knowing it was the prussian pressed harder and french army began to retreat, the last real force that remained to Napoleon were two battalions of the Old Guard stationed at the rear of the battle line so that Napoleon could rally his army in an eventual retreat but as retreat turned into rout, they too were forced to withdraw. The funny part is that even the generals of the Seventh Coalition army didn't expected to win that battle XD I don't exactly remember the title but it was an history assay should still have the book somewhere lol...

yeah he was one of the greatest leader in my country.