The Prussian army began arriving on the Waterloo battlefield at about 4:30 p.m. on 18 June 1815. The first major Prussian intervention came when General Friedrich von Bülow’s IV Corps attacked the French position around the village of Plancenoit and the French right flank.
The timing was crucial. Napoleon had expected Marshal Grouchy to prevent the Prussians from joining Wellington, but Grouchy’s forces did not reach Waterloo in time. Bülow’s arrival forced Napoleon to divert troops, including elements of the Imperial Guard, away from the main Anglo-allied line. This weakened the French position just as Wellington was trying to hold against renewed attacks.
More Prussian formations reached the battlefield later. Zieten’s I Corps and parts of Pirch I’s II Corps became engaged at approximately 6:00 p.m., increasing pressure on the French right and rear. The Prussian arrival did not instantly decide the battle, but it steadily transformed Wellington’s defensive struggle into a coordinated coalition offensive.