Throughout the late afternoon, the Prussian I Corps (Zieten's) had been arriving in greater strength in the area just north of La Haie. General Müffling, the Prussian liaison to Wellington, rode to meet Zieten.

Zieten had by this time brought up the Prussian 1st Brigade (Steinmetz's), but had become concerned at the sight of stragglers and casualties from the Nassau units on Wellington's left and from the Prussian 15th Brigade (Laurens'). These troops appeared to be withdrawing and Zieten, fearing that his own troops would be caught up in a general retreat, was starting to move away from Wellington's flank and towards the Prussian main body near Plancenoit. Zieten had also received a direct order from Blücher to support Bülow, which Zieten obeyed, starting to march to Bülow's aid.

Müffling saw this movement away and persuaded Zieten to support Wellington's left flank. Müffling warned Zieten that "The battle is lost if the corps does not keep on the move and immediately support the English army." Zieten resumed his march to support Wellington directly, and the arrival of his troops allowed Wellington to reinforce his crumbling centre by moving cavalry from his left.

The French were expecting Grouchy to march to their support from Wavre, and when Prussian I Corps (Zieten's) appeared at Waterloo instead of Grouchy, "the shock of disillusionment shattered French morale" and "the sight of Zieten's arrival caused turmoil to rage in Napoleon's army". I Corps proceeded to attack the French troops before Papelotte and by 19:30 the French position was bent into a rough horseshoe shape. The ends of the line were now based on Hougoumont on the left, Plancenoit on the right, and the centre on La Haie.