question: How did Frederick protect East Prussia when he went to invade Saxony ?
answer: leaving Field Marshal Hans von Lehwaldt in East Prussia to guard against Russian invasion from the east
turker_answer: Frederick protected East Prussia when he went to invade Saxony by leaving Field Marshal Hans von Lehwaldt in East Prussia to guard against Russian invasion from the east .
rule-based: Frederick protected East Prussia when he went to invade Saxony by leaving Field Marshal Hans von Lehwaldt in East Prussia to guard against Russian invasion from the east .