Complex sentence: Frederick the Great brought success to the Prussian arms, his successors lacked his talent, so generalship in the Army declined, even though they were assisted by a Quartermaster General Staff of adjutants and engineers established by Frederick the Great.
Split into simpler sentences: Frederick the Great brought success to the Prussian arms, his successors lacked his talent, so generalship in the Army declined. They were assisted by a Quartermaster General Staff of adjutants and engineers established by Frederick the Great.