Battle of Waterloo
Prelude: On 13 March 1815, six days before Napoleon reached Paris, the powers at the Congress of Vienna declared him an outlaw. Four days later, the United Kingdom, Russia, Austria, and Prussia mobilised armies to defeat Napoleon. Critically outnumbered, Napoleon knew that once his attempts at dissuading one or more members of the Seventh Coalition from invading France had failed, his only chance of remaining in power was to attack before the coalition mobilised.
Had Napoleon succeeded in destroying the existing coalition forces south of Brussels before they were reinforced, he might have been able to drive the British back to the sea and knock the Prussians out of the war. Crucially, this would have bought him time to recruit and train more men before turning his armies against the Austrians and Russians.
An additional consideration for Napoleon was that a French victory might cause French-speaking sympathisers in Belgium to launch a friendly revolution. Also, coalition troops in Belgium were largely second line, as many units were of dubious quality and loyalty.
The initial dispositions of Wellington, the British commander, were intended to counter the threat of Napoleon enveloping the Coalition armies by moving through Mons to the south-west of Brussels. This would have pushed Wellington closer to the Prussian forces, led by Gebhard Leberecht von Blücher, but might have cut Wellington's communications with his base at Ostend. In order to delay Wellington's deployment, Napoleon spread false intelligence which suggested that Wellington's supply chain from the channel ports would be cut.
By June, Napoleon had raised a total army strength of about 300,000 men. The force at his disposal at Waterloo was less than one third that size, but the rank and file were mostly loyal and experienced soldiers. Napoleon divided his army into a left wing commanded by Marshal Ney, a right wing commanded by Marshal Grouchy and a reserve under his command (although all three elements remained close enough to support one another). Crossing the frontier near Charleroi before dawn on 15 June, the French rapidly overran Coalition outposts, securing Napoleon's "central position" between Wellington's and Blücher's armies. He hoped this would prevent them from combining, and he would be able to destroy first the Prussian army, then Wellington's.
Only very late on the night of 15 June was Wellington certain that the Charleroi attack was the main French thrust. In the early hours of 16 June, at the Duchess of Richmond's ball in Brussels, he received a dispatch from the Prince of Orange and was shocked by the speed of Napoleon's advance. He hastily ordered his army to concentrate on Quatre Bras, where the Prince of Orange, with the brigade of Prince Bernhard of Saxe-Weimar, was holding a tenuous position against the soldiers of Ney's left wing. Prince Bernhard and General Perponcher were by all accounts better informed of the French advance than other allied officials and their later initiatives to hold the crossroads proved vital for the outcome. General Constant de Rebeque, commander of one of the Dutch divisions, disobeyed Wellington's orders to march to his previous chosen concentration area around Nivelles, and decided to hold the crossroads and send urgent messages to the prince and Perponcher. This fact shows how little Wellington believed in a fast French advance towards Brussels. He did not believe in recent intelligence given to him by General Dörnberg, one of his intelligence officials warning him of numerous French outposts south of Charleroi as well as some reports sent by the intelligence of the Prussian 1st corps. Had these two generals obeyed his orders, Quatre-Bras in all probability would have fallen to the French giving them time to support Napoleon's attack on the Prussians in the Sombreffe area via the fast, cobbled road, and the history of the campaign would have been significantly different.
Ney's orders were to secure the crossroads of Quatre Bras so that he could later swing east and reinforce Napoleon if necessary. Ney found the crossroads lightly held by the Prince of Orange, who repelled Ney's initial attacks but was gradually driven back by overwhelming numbers of French troops in the Battle of Quatre Bras. First reinforcements, and then Wellington arrived. He took command and drove Ney back, securing the crossroads by early evening, too late to send help to the Prussians, who had already been defeated.
Meanwhile, on 16 June, Napoleon attacked and defeated Blücher at the Battle of Ligny, using part of the reserve and the right wing of his army. The Prussian centre gave way under heavy French assaults, but the flanks held their ground. The Prussian retreat from Ligny went uninterrupted and seemingly unnoticed by the French. The bulk of their rearguard units held their positions until about midnight, and some elements did not move out until the following morning, ignored by the French.
Crucially, the Prussians did not retreat to the east, along their own lines of communication. Instead, they, too, fell back northwards parallel to Wellington's line of march, still within supporting distance and in communication with him throughout. The Prussians rallied on Bülow's IV Corps, which had not been engaged at Ligny and was in a strong position south of Wavre.
With the Prussian retreat from Ligny, Wellington's position at Quatre Bras was untenable. The next day he withdrew northwards, to a defensive position that he had reconnoitred the previous year—the low ridge of Mont-Saint-Jean, south of the village of Waterloo and the Sonian Forest.
Napoleon, with the reserves, made a late start on 17 June and joined Ney at Quatre Bras at 13:00 to attack Wellington's army but found the position empty. The French pursued Wellington's retreating army to Waterloo; however, due to bad weather, mud and the head start that Napoleon's tardy advance had allowed Wellington, there was no substantial engagement, apart from a cavalry action at Genappe.
Before leaving Ligny, Napoleon had ordered Grouchy, who commanded the right wing, to follow the retreating Prussians with 33,000 men. A late start, uncertainty about the direction the Prussians had taken, and the vagueness of the orders given to him meant that Grouchy was too late to prevent the Prussian army reaching Wavre, from where it could march to support Wellington. More importantly, the heavily outnumbered Prussian rearguard was able to use the River Dyle to fight a savage and prolonged action to delay Grouchy.
As 17 June drew to a close, Wellington's army had arrived at its position at Waterloo, with the main body of Napoleon's army in pursuit. Blücher's army was gathering in and around Wavre, around 8 miles (13 km) to the east of the town. Early the next morning, Wellington received an assurance from Blücher that the Prussian army would support him. He decided to hold his ground and give battle.

Armies: Three armies participated in the battle: Napoleon's Armée du Nord, a multinational army under Wellington, and a Prussian army under General Blücher.
The French army of around 74,500 consisted of 54,014 infantry, 15,830 cavalry, and 8,775 artilleries with 254 guns. Napoleon had used conscription to fill the ranks of the French army throughout his rule, but he did not conscript men for the 1815 campaign. His troops were mainly veterans with considerable experience and a fierce devotion to their Emperor. The cavalry in particular was both numerous and formidable, and included fourteen regiments of armoured heavy cavalry, and seven of highly versatile lancers who were armed with lances, sabres and firearms.
However, as the army took shape, French officers were allocated to units as they presented themselves for duty, so that many units were commanded by officers the soldiers did not know, and often did not trust. Crucially, some of these officers had little experience in working together as a unified force, so that support for other units was often not given.
The French were forced to march through rain and black coal-dust mud to reach Waterloo, and then to contend with mud and rain as they slept in the open. Little food was available, but nevertheless the veteran soldiers were fiercely loyal to Napoleon.
In December 1814, the British Army had been reduced by 47,000 men. This was largely achieved by the disbandment of the second battalion of 22 infantry regiments. Wellington later said that he had "an infamous army, very weak and ill-equipped, and a very inexperienced Staff". His troops consisted of 74,326 men: 53,607 infantry, 13,400 cavalry, and 5,596 artillery with 156 guns plus engineers and staff. Of these, 27,985 (38%) were British, with another 7,686 (10%) from the King's German Legion (KGL). All of the British Army troops were regular soldiers, and the majority of them had served in the Peninsula. Of the 23 British line infantry regiments in action, only four (the 14th, 33rd, 69th, and 73rd Foot) had not served in the Peninsula, and a similar level of experience was to be found in the British cavalry and artillery.   Chandler asserts that most of the British veterans of the Peninsular War were being transported to North America to fight in the War of 1812.