Complex sentence: Advance troops reached shores of Lake Onega on 24 September and the town of Petrozavodsk was captured on 1 October after the Soviets withdrew from the town to avoid encirclement.
Split into simpler sentences: Advance troops reached the shores of Lake Onega on 24 September. The town of Petrozavodsk was captured on 1 October after the Soviets withdrew to avoid encirclement.