Brussels in June 1815 was a strange place: a glittering rear-area to a war that had not yet started. Napoleon had escaped Elba in March, the Bourbon king Louis XVIII had fled across the border, and the Anglo-allied army under Wellington had taken up positions in the Low Countries alongside the Prussians under Blücher. The Duke of Richmond, commanding a reserve force in Brussels, had rented a house on the Rue des Cendres. His wife Charlotte, the Duchess, had been planning her ball for weeks. The British and Belgian aristocracy had followed the army to Brussels for the season. The atmosphere was odd: dinner parties, picnics, theatre, all carrying on while sixty miles south, on the road from Charleroi, French columns were already moving.

The room itself was no grand hall. Sir William Fraser, who investigated the site in 1888, concluded the dancing took place not in the main house but in a coach maker's depot that backed onto it, a room about 120 feet long, 54 feet broad, and only 13 feet high. The low ceiling embarrassed Byron's later poetic line about 'that high hall,' but it could pack in a crowd. Hundreds of names appeared on the invitation list: the Prince of Orange, the Duke of Brunswick, the Earl of Uxbridge, Sir Thomas Picton, Lord FitzRoy Somerset, scores of colonels and captains and ensigns. The guests of honour were ostensibly the great and the good of Brussels society. The actual audience, watching through doorways, were aides hoping for news.

Wellington arrived between eleven and midnight, conspicuously calm. He had been receiving dispatches all evening. Shortly before supper at one in the morning, Lieutenant Henry Webster brought a message for the Prince of Orange; the Prince handed it across to Wellington, who pocketed it unopened. When he finally read it, the message reported that the Prussians had been pushed back from Fleurus, north-east of Charleroi. The French were across the Sambre. Wellington asked the Prince to return to his headquarters and then, with practiced sangfroid, went into supper between Lady Frances Wedderburn-Webster and Lady Georgiana Lennox. A second dispatch arrived in whispers: the French had pushed up the Charleroi road nearly as far as Quatre Bras. Wellington kept eating. He talked. He made small talk for another twenty minutes. Only then did he rise, ask the Duke of Richmond for a map, and famously remark, by one account: 'Napoleon has humbugged me, by God.'

Word spread through the dancers. Officers slipped away, some still in dancing slippers, some in dress uniform with no time to change. Katherine Arden, daughter of Baron Alvanley, left a description of the rest of the night that haunts every retelling: the carriages, the embraces, the husbands and brothers riding south in the dark. The Duke of Brunswick rode straight from the ball to Quatre Bras and was dead by the next afternoon. Ensign Lord Hay, aide to Major-General Maitland, was also killed at Quatre Bras. Colonel John Cameron of the 92nd Highlanders, killed there too. Two days later at Waterloo, the casualty list lengthened: Sir Thomas Picton shot through the head leading his division, Sir William Ponsonby cut down with the cavalry, Sir Alexander Gordon mortally wounded delivering messages, William Howe De Lancey struck by a cannonball at Wellington's side. Lord FitzRoy Somerset, who would later become Lord Raglan, lost his arm. The Earl of Uxbridge lost his leg.

Nineteenth-century novelists understood instantly what they had been given. Byron wove the ball into Childe Harold's Pilgrimage: 'There was a sound of revelry by night.' Thackeray placed Becky Sharp and George Osborne in its crowd in Vanity Fair, with Osborne dancing toward a death on the field outside. Millais painted The Black Brunswicker in 1860, modelling the parting of an officer and his beloved. Henry Nelson O'Neil's 1868 Before Waterloo crowded the staircase with departing soldiers. When Hollywood reached for the scene in the 1935 Becky Sharp, the first full-length three-strip Technicolor film, the moment the red coats poured out of the ballroom became the sequence that made critics believe colour cinema could carry drama. The image is enduring because it is true. People danced. Then they rode out and died. Brussels has hosted a commemorative ball on June 15 ever since 1965.