Complex sentence: For many years, he taught the special subject in military history at Oxford with Professor N.H. Gibbs, which involved using the War of the Second Coalition as a case study for examining the theories of Carl von Clausewitz.
Split into simpler sentences: For many years, he taught the special subject in military history at Oxford with Professor N.H. Gibbs. This course of study involved using the War of the Second Coalition as a case study for examining the theories of Carl von Clausewitz.