Racing to strike before the Prussian and Anglo-allied armies could unite, Napoleon pushed roughly 124,000 troops across the Sambre near Charleroi. The move caught the allies partially off guard and triggered the frantic marches and skirmishes at Ligny and Quatre Bras that preceded Waterloo on June 18. It was Napoleon's last offensive campaign, a gamble to shatter his enemies before Europe's full coalition could crush him.
After escaping exile on Elba and reclaiming Paris without firing a shot — the famous 'Flight of the Eagle' — Napoleon knew he had perhaps weeks before Britain, Prussia, Austria, and Russia mobilized overwhelming numbers against him. His only hope was to strike the nearest allied forces, Wellington's Anglo-allied army and Blücher's Prussians, before they combined.
Crossing the Sambre in the pre-dawn hours, French columns moved with a speed and secrecy that briefly wrong-footed the allies; Wellington reportedly learned of the advance only late in the day and admitted Napoleon had 'humbugged' him. Fighting broke out almost immediately at Charleroi and along the river approaches.
What followed was a compressed, brutal campaign: Napoleon defeated Blücher's Prussians at Ligny on the 16th while Wellington fought a bloody draw at Quatre Bras the same day, before the decisive collision at Waterloo two days later. The Sambre crossing is easy to overlook next to Waterloo's drama, but it was the opening move of the campaign that ended the Napoleonic Wars and reshaped Europe's map for a century.
Key people: Napoleon Bonaparte, Duke of Wellington, Gebhard von Blücher