Complex sentence: He received no formal education, but served in the court of Pippin the Younger until he left to serve in Pepin's army, serving eight years in the field in Lombardy, Saxony, and the Aquitaine.
Split into simpler sentences: He received no formal education, but served in the court of Pippin the Younger until he left to serve in Pepin's army. He spent eight years in the field in Lombardy, Saxony, and the Aquitaine.