Kleist began his 20-year military career at the gendarmes in Berlin, on garrison duty. In 1756, at the onset of the Seven Years' War, Frederick II transferred him to Hussar Regiment Szekely. On 26 August 1756, when the Prussian Army invaded Saxony, the regiment was part of the left column led by the Augustus William, Duke of Brunswick-Bevern. Initially concentrated in the area of Lübben, the column advanced through Lusatia via Hoyerswerda and Bautzen; by 8 September, the column had reached Hohenstein then marched toward the Elbe near Pirna. On 1 October, eight of its squadrons took part in the action at Lobositz, covering the left flank. It did not participate in the second attack against Browne's Austrians.