Context
Returning from Elba during the Hundred Days, Napoleon had to face a coalition formed in Vienna. He decided to strike the British and Prussian armies in Belgium before they could join forces.
Forces
France: About 72,000 men. The Imperial Guard, d'Erlon's, Reille's and Lobau's corps. Marshal Ney commanded the left wing.
Coalition: About 68,000 under Wellington (British, Dutch, Hanoverians) entrenched at Mont-Saint-Jean. About 50,000 Prussians under Blücher marching towards the battlefield.
Course of the battle
On the morning of 18 June, rain delayed the start. Napoleon launched frontal attacks on the allied centre (Hougoumont farm, La Haye Sainte). French cavalry charges failed against British squares. Blücher's Prussians gradually arrived on the eastern flank. In late afternoon, the Imperial Guard attacked; it was repulsed. The rout became general.
Consequences
Napoleon abdicated on 22 June. Louis XVIII was restored. Waterloo sealed the end of the First Empire and opened the era of the Congress of Vienna.