The Duke of Wellington had ridden over this ground the year before, and he had not forgotten it. The low ridge running east to west near the village of Waterloo gave him what he wanted: a reverse slope on which to hide his troops from French artillery, and three farms - Hougoumont on the right, La Haye Sainte in the centre, and Papelotte on the left - that anchored his line like buttresses. He stationed his men in a single line just behind the crest, where the French could neither see them nor easily shell them. He had 68,000 soldiers under his command, a polyglot army he himself called "infamous, very weak and ill-equipped." Many of his Dutch and Belgian troops had worn French uniforms not long before. Wellington's calculation was simple and brutal: hold the ridge until the Prussians could arrive from the east. Everything else was bluff.

The fighting began at Hougoumont, a walled chateau on Wellington's right flank, around 11:30 in the morning. Napoleon meant the attack as a feint to draw Wellington's reserves; instead it consumed thousands of French infantry all day long. Around one o'clock, a French sergeant called Legros - a giant of a man known to his comrades as l'Enfonceur, "the Smasher" - hacked through a north gate with an axe and led perhaps forty men into the courtyard. Lieutenant Colonel James Macdonell of the Coldstream Guards and a small group of his men forced the gate shut against the press of French bodies and killed every Frenchman trapped inside, except a young drummer boy whom they spared. Wellington said later that the outcome of Waterloo turned on the closing of the gates at Hougoumont. He may have meant it literally.

The struggle for the farmhouse of La Haye Sainte, at the very centre of Wellington's line, lasted nearly the whole day. Four hundred soldiers of the King's German Legion - Hanoverians fighting for their exiled king - held its walls against repeated French assaults. They fought through the morning, through the great French cavalry charges of mid-afternoon when Marshal Ney sent some 9,000 horsemen in wave after wave against Wellington's infantry squares, and on into the evening. Only when their ammunition finally ran out, around six o'clock, did the farmhouse fall. Their commander, Major Georg Baring, escaped with a handful of survivors. By then the centre of Wellington's line was so thin that he is said to have prayed, quietly, for night or for Blücher.

Field Marshal Gebhard Leberecht von Blücher was 72 years old and had been knocked unconscious beneath his dead horse at the Battle of Ligny two days earlier. He woke up, drank some schnapps mixed with garlic against the pain, and gave the order that decided the campaign: march to Wellington, not back to Prussia. His staff thought he was mad. The roads were knee-deep in mud, the men exhausted, the French cavalry of Marshal Grouchy somewhere behind them. But Blücher had given his word. Through the long afternoon his Prussians pushed west, four corps of them, around 50,000 men, dragging their guns through bogs. They began arriving on Napoleon's right flank near the village of Plancenoit around 4:30 in the afternoon. By evening, Napoleon was fighting on two fronts at once, and his reserves were melting away to hold the Prussians off.

Around 7:30 in the evening, with the light fading, Napoleon committed his last reserve: the Imperial Guard, the soldiers who had never been defeated. They climbed the ridge in column, shoulder to shoulder, eight battalions of veterans who had marched with Napoleon from Egypt to Moscow. Wellington had hidden the British Foot Guards lying flat in the wheat. At point-blank range he gave the order most British soldiers remembered as: "Now, Maitland! Now's your time!" The redcoats stood up out of the grain and fired a volley into the head of the column from forty paces. The Guard reeled, fired back, and then - for the first time anyone could remember - they broke. A cry went up along the French line: La Garde recule. The Guard is falling back. Within minutes the entire French army was in flight. At Plancenoit, a square of grenadiers refused to surrender; legend has it that their commander, General Cambronne, replied to a British demand with the single word merde, though the speech later attributed to him - La Garde meurt, elle ne se rend pas, "the Guard dies, it does not surrender" - is what the schoolbooks remember.

By dawn on 19 June, the wheat fields south of Mont-Saint-Jean were carpeted with the dead and the dying. Around 47,000 men lay killed or wounded across roughly four square kilometres - one casualty for every three paces of ground. Some bodies were not buried for weeks; for years afterwards, farmers ploughing the field turned up bones, buttons, and teeth, the last of which were collected and sold to British dentists for use in dentures called, with bleak humour, "Waterloo teeth." Napoleon abdicated four days later and was sent to St. Helena, where he died. Wellington wept when the casualty lists were read to him and never spoke of the day with any pride. "Nothing except a battle lost," he wrote, "can be half so melancholy as a battle won."