Also having read a great deal about the Waterloo Campaign I'll contribute.
French CommandersWhen Napoleon returned from Elba, he had less of his Marshals than he had done before, the best available were him in terms of their ability are Ney, Davout, Mortier, and Grouchy who was made Marshal during the return of Napoleon.
Many good comanders were still available as well who were Generals, such as d'Erlon, but Napoleon was quite short on men he could rely on to do a task exceptionally, he needed a Minister of War, Davout despite being one of the best of Napoleon's Marshals was best qualified to be minister of war than anyone else, so he did not take part in the campaign.
Tragically, Mortier could not join the campaign because of disease, so he was unable to take part.
Ney, as seen in his actions during the campaign, is believed to of been distracted during the campaign, he was not the decisive leader he was before, this is probably due to his personal conflicts about betraying the King whom he'd sworn an oath too, he now knew if Napoleon fell, he would fall with him. As a result, he was reckless and indescisive and Quatre Bras could have been an easy Victory had Ney been so indescisive.
Grouchy, many have blamed Grouchy for the defeat at Waterloo for not arriving to Napoleon's aid, however he was a very good commander and defeated the Prussians at Wavre
Dat RainThe downpour of rain on the night of the 17th of June practically saved the Allies in a sense, as a result Napoleon was unable to attack at 9am as he had planned, he had to attack after 11am, this delay was crucial because Napoleon knew the Prussians would be arriving which was why he wanted to destroy Wellington quickly and as soon as possible. The rain had taken his speed and mobility to set up his troops and artillery.
Hougoumont, La Haye Sainte and PapelotteThere is no question that the attempts to take these vital strategic points by the French were inadequate, almost no artillery was used to break the defences of these farm houses and as a result, French lives were spent in vain to breakthrough, and when breakthroughs did happen, the French were too slow to take the advantage. For example, a Lieutenant called Bonnet (or Legros?) used an engineer axe to break down the Northern gate of Hougoumont and led as small valiant group inside, but they were killed by a lethal musket volley. Had the storming party been bigger, had the French taken quick advantage, the Northern gate could have easily been overwhelmed at this point, but too late, the gates were closed. Hougoumont was also supposed to be a diversionry attack to have Wellington take men away from his centre (where Napoleon planned to attack) and to the Allied right, which in this respect was a success, however, Reille, commander of the II Corps failed to keep the attack diversionary, and the number of troops committed became far to much. Contary to many British sources, which often over estimate the numbers of troops committed to the attacks against them.
La Haye Sainte, had it fallen earlier in the battle than it did at 6:30pm, then the battle too could have been very different, it would have allowed the French artillery to advance earlier to fire at the Allies at close range, however, for the same reasons as for Hougoumont and Papelotte, the organisation for these attacks were lacking.
d'Erlon's AttackThere is a lot of controversy surrounding the attack of d'Erlon's I Corps' attack. Having not taken part in either Ligny of Quatre Bras (due to conflicting messages from Napoleon and Ney) d'Erlon was able to commit 20,000 men to his attack, however the comlumn of attack used has been seen as very contraversal. Instead of the common French practice to form columns of battalions, instead the attacks were done by whole divisions, this was where the entire divisions battalions would form in three ranks, and be arranged one battalion behind another, making the column much wider than long. The reason for this being controversal was because it has been seen as unwieldly, cumbersome and impossible to control, however lets look back. Many of the French Generals had fought Wellington in Spain, and knew what the British troops in the Allied army were capable of against the normal French column, so this formation of a battalion wide column was probably a comprimise and an attempt to counter the British infantry two ranked line. Had the attack been successful, either Ney or d'Erlon would have been seen as a hero.
One of the divisions was sent to take La Haye Sainte, the rest marched up the ridge to the right of it if one were looking from the French ridge, it was Pegot's Marcognet's and Donzelot's divisions that advanced.
So how did the attack fail? Many British historians and officers have tried to suggest that it was the infantry who inflicted the columns with much death and that the columns had already started retreating before the charge of the British cavalry, however there is much more sufficient evidence that the attack from d'Erlon's Corps was on the verge of success, what is certain is that the French columns forced the first line of the Dutch-Belgian troops to fall back before being supported by Picton's Division. There is sufficient evidence to suggest that the French divisional columns were actually repulsing Picton's division, this is because most likley that the fire from the British and Allied troops was equal, rather than superior to the French fire.
So at this point, with the Division's of d'Erlons Corps were succeeding and breaking apart the Allied line, the timley charge by the British cavalry changed that however, what is certain is that the French were caught completley by surprise by the British cavalry, panic spread quickly and soon what had been organised troops close to possible victory had become a rabble, fleeing back towards the French ridge. This is another reason for why the divisional columns have been condemned, they offered very little defence against cavalry, the battalions had no room to form square because of the other batalions in front or behind them.
It appears that there was no, or very little French cavalry supporting the attack, it appears that the 1st Cavalry division of d'Erlons Corps had not advanced to support the attack, why they did not is very much in doubt.
So in truth, the French did not "come on in the same old style" as Wellington wrote in his report of the battle.
Significance of the PrussiansThere can be no doubt the Prussians were as essential to the Allied army led by Wellington, without their intervention it is very unlikley Wellington would have won, and in fact had Blucher reported that he would not be able to aid Wellington before the battle, Wellington is unlikley to have tried to fight Napoleon at all.
The Prussians arrived at the battle around 4pm, Napoleon committed Lobau's VI Corps was commited, aroung 10,000 men, to holding up the first Prussian Corps (and did it very well to the best of their ability). Then the Young Guard Division was commited to defend Plachenoit, numbering around 4,500, Subervie's and Domon's Cavalry divisions, numbering around 2,300 men and finally two battalions of the Old Guard numbering about 1,100. This is a total of nearly 18,000 men commited to fighting the Prussians.
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The French Cavalry AttacksThe French cavalry performed excellently during the campaign, capturing around at least 4 British colours (To the Allied two Eagles).
The ordering of committing all the cavalry against solid Infantry squares at Waterloo is often blamed on Marshal Ney, for mistaking Allied re-deployment for signs of a retreat. However, at that point of the battle there were no other infantry troops available to attack the Allied line, d'Erlons Corps was regrouping after their repulse from the British cavalry, half of the II Corps under Reille was committed to the fight for Hougoumont, so Ney, who was commanding the army while Napoleon was organising the delaying of the Prussians, had practically no alternative but to use the cavalry.
However, Ney again showed the signs of rashness, as a commander he had the duty of overseeing the charges and giving what support needed to be given, however instead he was leading from the front charging the squares. Brave this may of been, it was not the behavior of a commander of an army, he was too late to send for artillery and Infantry support, by the time the Infantry was prepared (some troops from Reille's II Corps) the French cavalry was to batterred and combat ineffective for combined attacks to make a telling effect, the Infantry sent was repulsed. However, after the fall of La Haye Sainte, French artillery was able to get close enough to the Allied squares to fire canister and case shot, which devasted many of the squares, but this was not enough to break the French line.
The Attack of the Middle GuardContary to many British estimations, the attack of the Middle Guard consisted of just 5 Battalions. These battalions did not attack in to large echelons as is popular belief, they advanced towards the Allied line in square of battalions, this was to protect them from cavalry as they had no wish to share the fate of d'Erlon's Corpss earlier in the day. What is clear from eye-witness accounts is that French musket fire effectivness during this attack was very evident. In fact nearer the centre of the Allied line, to the right of the Middle Guard's attack, there is evidence that shows the battered Allies were crumbling from the effective fire of the veteran Guardsman and were starting to retreat in disorder, as an officer of the 33rd reported. it was the support of the Dutch-Belgin and other Allied battalions behind these battered brigades that later repulsed the attacks on the right, at the same time, the British Guards managed to repulse attack against them with the help of the 52nd, who despite taking heavy casualties from the French Guard's fire, managed to flank the furthest left square.
Simply, the 5 battalions Napoleon had to attack the Allies with was simply to small a number to break through the centre, despite them being of the Guard.
I expect I could write tons more but I'm knackered, enjoy