Complex sentence: Grenadier Regiment 10 had been subjected to a "torrent" of fire overnight, which had forced the German infantry to shelter in mine galleries, a gas bombardment was synchronised with the French infantry attack and the mine explosions at killed many of the sheltering troops.
Split into simpler sentences: Grenadier Regiment 10 had been subjected to a "torrent" of fire overnight, which had forced the German infantry to shelter in mine galleries. A gas bombardment was synchronised with the French infantry attack and the mine explosions at killed many of the sheltering troops.