waterloo

[matteus the inbred]

too many reasons, depending on who you read!
it rained heavily the night before, making for a late start to allow the ground to dry out for moving artillery, the last thing Napoleon needed.
he failed (for many reasons, not all his fault) to prevent the Prussians from rejoining the battle.
he appointed the wrong Marshals in the wrong commands (revisionists mainly blame Ney and of course Grouchy for making poor tactical decisions; but of course Napoleon appointed them to their respective commands in the first place, and this wasn't the first time he'd picked the wrong man for the job)
he had some kind of tactical 'battle fatigue', to which he was occasionally prone (such as at Borodino) where he couldn't be bothered to do anything clever and just slugged it out, thus underestimating the British infantry on a battlefield chosen by Wellington. there is no reason for Napoleon not to have been familiar with Wellington's troop quality or expertise at using terrain as his Marshals would have told him about this (and there is anecdotal evidence that they did).
it's hard to say whether the quality of the French army and particular their commanders was poor or not poor comparatively, after all Napoleon had accomplished great things with a very low quality army in 1814...he was certainly missing the likes of Davout and Murat, anyway.

even so, given his strategic and (up till now) tactical brilliance in the 1815 campaign, it's odd that he failed to defeat an army that was not anywhere near the best that Wellington ever commanded. it's even possible to argue that Waterloo was lost before it was even fought, as the failure to detroy the Prussian army at Ligny and damage the British at Quatre Bras made fighting against the clock at Waterloo inevitable. personally, i think with his 1805-1812 army and its commanders Napoleon would have won fairly easily.

[Templar Knight]

His attack on the Château d'Hougomont failed, which set the pattern for the rest of the day. He hoped that an attack here would draw out Wellingtons reserves. Its stubborn defence continued to draw thousands of valuable French troops which included:

Detachments under the command of Jerome Napoleon's brother
The divisions of Foy, Guilleminot and Bachelu
Nearly the entire corps of Reille
Kellermann's cavalry corps
Bauduin's brigade failed to enter Hougomont on the north side
Soye's brigade managed a small breach on the south side could not exploit it.

Apparently: Napoleon's hemorrhoids contributed to his defeat at Waterloo. Apparently his piles prevented him from surveying the battlefield on horseback.

Edit (too fast IA )

[matteus the inbred]

yeah, the 'illness of some kind' (piles/indigestion/fainting) appears to be fairly substantiated and effectively turned command over to Marshal Ney, who wasn't up to the job despite being insanely brave.
Grouchy's failure may be compared to that of Bernadotte at Jena-Auerstadt, although Napoleon had no Davout to pull his irons out of the fire this time.

Interesting trivia, Napoleans army still outnumbered the Allies even as they routed.

i didn't know that one, InsaneApache, very interesting! psychological collapse due to the failure of the Guard. la garde recule!

[VAE VICTUS]

and i remember in cromwell, cornwall whatever author writes the sharpe series,
something about a prince of orange who did something dumb but cant remember what.
anything like that happen, or was it made up?

[matteus the inbred]

Bernard Cornwell, Sharpe's Waterloo...it may be true; the Prince of Orange held a divisional command at Waterloo. he was supposed to have exposed infantry in line to cavalry attack (which equals lots of dead infantry) at least twice during the battle and once at Quatre Bras as well...there is some evidence for this (it certainly happened, but whose fault it was is not clear). his appointment was a political one, in order to make Dutch-Belgian troops available to the allies during the campaign.

like Ney, there's no doubting his bravery (he led a cavalry charge and was later wounded by a sniper for getting recklessly close to the fighting at La Haye Sainte) but his tactical judgement was at best questionable.

appointments like his, and the low quality of the troops he brought with him (some of them had fought for Napoleon only the year before!) are one of the reasons why Waterloo was in no way unwinnable for the French despite the race against time and (overall, counting the Prussians) superior numbers.

[Watchman]

Ney tends to get rated as a "excellent subordinate, poor commander" sort - he ended up with responsibilities above his abilities, which rarely turns out well. Not that he'd have been the first capable officer in history who ended up messing up due to that...

I've also read that old "Bonie" himself just wasn't what he'd been anymore either - too much stress, hard campaigns and less-than-ideal lifestyle left him physically exhausted and with assorted health issues (he'd had those for a while, I understand), and started blunting his mental edge too if only out of sheer fatigue. Someone described it as "his opponents may have improved considerably by Waterloo, but he had also gotten worse" or something along those lines. Given his rather centralized way of running the army that would certainly have been a major drawback.

[English assassin]

To be fair to Ney he was a Cavalry commander put in charge of a whole army corps. I've read that he was so carried away with his cavalry charge that he forgot that his infantry would not move up behind unless he ordered it.

With the result that the British infantry were forced into square, with the gunners inside, but no one came up behind the French cavalry to capture the British guns, turn them on the British squares, or engage the squares with musket fire. Had they done so...well, as the Iron Duke himself said, it was a close run thing.