The Prussians had come

From Waterloo A Near Run Thing by David Howarth. Page 153.

From time to time all through the afternoon, the outcome of the battle had hung by the thinnest threads. Now for some minutes everything depended on this solitary Prussian, his horse and his powers of persuasion. He overtook the retreating troops, and by luck he soon found von Zieten. The appearance of a general all alone, on a sweating horse and in a state of desperate anxiety, made von Zieten halt. He was reluctant to countermand his order and turn his men round again. ‘You are mistaken,’ Muffling assured him, ‘the British are not in retreat, they are standing fast. But the battle is lost unless you come back at once.’ At last, von Zieten turned his men; and in doing so he turned the tide which had reached its lowest ebb. In fifteen minutes, his advance guards were in action at the end of Wellington’s line. British cavalry which had been stationed there all day was moving along the back of the ridge to support the wavering centre. And the news was spreading along the line from man to man and from regiment to regiment: the Prussians had come.