Complex sentence: The Reichswald ridge is a glacial remnant which, when wet, easily turns to mud and at the time of the operation, the ground had thawed and was largely unsuitable for wheeled or tracked vehicles.
Split into simpler sentences: The Reichswald ridge is a glacial remnant which, when wet, easily turns to mud. At the time of the operation, the ground had thawed and was largely unsuitable for wheeled or tracked vehicles.