Winterfeldt was at Pirna with the king; when the Saxons surrendered there, Frederick considered incorporating the Saxon army wholesale into his own, and Winterfeldt advised him against this act. Frederick did it anyway. He accompanied Schwerin in the advance on Prague in 1757 and took a conspicuous part in the battle there, where the Saxon army deserted the Prussians by regiment. After the defeat of Kolin, however, Winterfeldt, whom Frederick seems to have regarded as the only man of character whom he could trust to conduct the more delicate and difficult operations of the retreat, found himself obliged to work in close contact with the king's brother, Prince William, Zieten and others of the men who considered him their enemy.