Complex sentence: The major battles on the Western Front had been fought in France, and the French countryside had been heavily scarred in the fighting; in 1918 during the German retreat, German troops devastated France's most industrialized region in the north-east.
Split into simpler sentences: The major battles on the Western Front had been fought in France, and the French countryside had been heavily scarred in the fighting. In 1918, during the German retreat, their troops devastated France's most industrialized region in the north-east.