Complex sentence: The young Gaston was sent as a subordinate officer to participate in the conflict on the side of the Spanish forces that consisted of more than 40,000 soldiers, against the Moroccan troops, who in turn had about 140,000 men.
Split into simpler sentences: The young Gaston was sent as a subordinate officer to participate in the conflict on the side of the Spanish forces. The Spanish military consisted of more than 40,000 soldiers, while the Moroccan troops numbered about 140,000 men.