Napoleon’s right wing was delayed away from Waterloo because Marshal Emmanuel de Grouchy was ordered to pursue the retreating Prussians. Grouchy’s force engaged the Prussians at Wavre instead of joining Napoleon’s main army at Waterloo.
After the French victory at Ligny on 16 June 1815, Napoleon expected the defeated Prussians to retreat eastward. On 17 June he detached Grouchy with the right wing—principally III and IV Corps—to follow them, while the rest of the army moved against Wellington.
The Prussians regrouped and marched toward Waterloo. Grouchy’s troops fought at Wavre on 18–19 June, winning a tactical success but failing to prevent the main Prussian army from reaching the battlefield. The delay mattered enormously: Prussian corps arrived during the afternoon and evening, attacked the French flank and helped bring about Napoleon’s defeat.