Complex sentence: In military terms, the area northwest of Breslau was predominantly flat; while the absence of steep hills made observation of an approaching enemy easy; the flatness precluded hiding maneuvers from one's enemies.
Split into simpler sentences: In military terms, the area northwest of Breslau was predominantly flat. While the absence of steep hills made observation of an approaching enemy easy, the flatness precluded hiding maneuvers from one's enemies.