Complex sentence: On October 1st, 1918 Whittlesey was given his orders: first, he was to advance north up the Ravine d’Argonne until it ended, at the Ravin d’Charlevaux.
Split into simpler sentences: On October 1st, 1918 Whittlesey was given his orders. First, he was to advance north up the Ravine d’Argonne until it ended and the Ravin d’Charlevaux.