First, Napoleon's army was vastly outnumbered. The combined Allied forces of the Seventh Coalition, led by the Duke of Wellington and the Prussian, Gebhard von Blucher, numbered nearly 120,000 men; Napoleon could only muster about 70,000. Napoleon went on the offensive, hoping to crush the Coalition forces before more troops--particularly from Russia and Austria--could be organized to invade France, hoping to defeat the British and drive them to the sea. Napoleon was able to defeat the Prussian forces at the Battle of Ligny on June 16, 1815, but the retreating Prussians managed to escape the pursuing French, reaching the town of Wavre, eight miles east of Waterloo, Belgium (then The Netherlands). Wellington's British and Dutch forces had already arrived at Waterloo, where they enjoyed a strong defensive position.

Napoleon's army consisted of veterans with no conscripts, and he had a strong cavalry that included armored heavy troops; Wellington's troops and staff were mostly inexperienced. Napoleon launched his initial assaults about 10 a.m. on June 18, but his troops were never able to sufficiently break through the well-defended Coalition positions. When von Blucher's Prussians arrived, they managed to pierce Napoleon's right flank, and he was forced to commit his reserves. Wellington then counter-attacked, and the French army was forced to retreat in disorder. In all, Napoleon suffered about 33,000 casualties, while the Coalition forces lost about 22,000 men.