Complex sentence: The campaign ended in complete disaster as Napoleon and his remaining forces retreated during the bitterly cold Russian winter, with sickness, starvation, and the constant harrying of Russian Cossack marauders and partisan forces leaving the Grande Armée virtually destroyed by the time it exited Russian territory.
Split into simpler sentences: The campaign ended in complete disaster as Napoleon and his remaining forces retreated during the bitterly cold Russian winter. Sickness, starvation, and the constant harrying of Russian Cossack marauders and partisan forces left the Grande Armée virtually destroyed by the time it exited Russian territory.