It was the battle that determined the shape of Europe for decades, and which led to the UK's hegemony on the world stage. It was fought on June 18th, 1815, at Waterloo. Wellington had, as always, inspected the battlefield beforehand, noting which low rises would conceal troops of his Anglo-Dutch army, and decided to give battle at the Mont-Saint-Jean escarpment. Napoleon had performed his usual manoeuvre of rushing his troops forward to attack his enemies one by one before their forces could combine. He defeated Marshall Blücher's Prussians at Ligny, before they could combine with Wellington's forces. Crucially, however, he overestimated his victory, and underestimated the moral courage of Marshall Blücher.
Blücher had retreated in good order and promised Wellington that the Prussians would join him at Waterloo before the day was out. Given that pledge, Wellington committed to battle and withstood repeated French attacks throughout the afternoon, with both sides sustaining heavy casualties. Realizing that the Prussians were approaching, Napoleon committed his Imperial Guard, the "Immortals," in a desperate last attack. Never before repulsed, the Imperial Guard retreated under heavy fire, even as Blücher's Prussians entered the field, with General Bülow breaking through on the French right flank. Wellington counter-attacked, sending the French into headlong retreat.
Blücher had delivered on his promise, and his entry onto the field late in the day, swung the battle. Napoleon fled in his carriage, leaving his personal possessions as well as his troops, bringing an end to French dreams of ruling Europe, and closing the page on the years of constant war that had followed the French Revolution. On 21 June, 1815, at London's East India Club, Major Henry Percy presented the Prince Regent with four captured French eagles and Wellington’s victory despatch from the Battle of Waterloo. The news was then announced from the balcony to the crowds that had gathered below. The room where this took place is now known at the Club as the "Waterloo Room".
Four decades of comparative peace ensued, marked by scientific and technological progress and economic expansion. The UK gained most from the victory. It put an end to centuries of Anglo-French warfare, and left Britain's navy ruling the high seas, protecting a worldwide empire "on which the sun never set." The Industrial Revolution, briefly interrupted by the needs of war, now accelerated, bring unparalleled material prosperity to the nation. Britain became, for a time, the workshop of the world, exporting the machines that powered the industries that other nations were developing in her wake.
Wellington himself, the last non-royal person awarded a dukedom, became a successful Prime Minister, helping to usher through the major reforms that transformed Britain into a modern power. At the Great Exhibition of 1851, the one that showed off Britain's technical, scientific and manufacturing expertise to the world, it is reported that the loudest cheer from the crowd was for the 82-year-old warrior himself. When he died the following year, Queen Victoria insisted on a lavish state funeral. Napoleon, meanwhile, had died in 1821, a solitary prisoner on the Atlantic island of Saint Helena.
It could easily have gone otherwise. Wellington said, "It has been a damned nice thing — the nearest run thing you ever saw in your life," (often summarized as "a damn close-run thing"). In his official dispatch, Wellington wrote that victory would not have been possible without the timely assistance of Marshall Blücher. It was the battle that day which determined that liberal Britain, rather than autocratic France, would come to dominate world culture, and that English would become the lingua franca of the modern world.