Query: How did Frederick protect East Prussia when he went to invade Saxony?
Context: Accordingly, leaving Field Marshal Count Kurt von Schwerin in Silesia with 25,000 soldiers to guard against incursions from Moravia or Hungary, and leaving Field Marshal Hans von Lehwaldt in East Prussia to guard against Russian invasion from the east, Frederick set off with his army for Saxony. The Prussian army marched in three columns.