Complex sentence: Later he attended a school at Péronne, founded by M. Ballue de Bellenglise, who followed the educational principles of Jean-Jacques Rousseau, where the boys were formed into clubs and regiments, and taught to play at politics and war.
Split into simpler sentences: Later he attended a school at Péronne, founded by "M. Ballue de Bellenglise", which was run according to the educational principles of Jean-Jacques Rousseau. Here, the boys were organised into clubs and regiments, and taught to play at politics and war.