Complex sentence: But there was little scope for the activities of a young and energetic subaltern, and, leaving the service in 1836, enlisted in the Carlist army fighting the First Carlist War in Spain.
Split into simpler sentences: But there was little scope for the activities of a young and energetic subaltern. Leaving the service in 1836, he enlisted in the Carlist army fighting the First Carlist War in Spain.