Complex sentence: Fuller is perhaps best known today for his "Nine Principles of War" which have formed the foundation of much of modern military theory since the 1930s, and which were originally derived from a convergence of Fuller's mystical and military interests.
Split into simpler sentences: Fuller is perhaps best known today for his "Nine Principles of War" which have formed the foundation of much of modern military theory since the 1930s. These were originally derived from a convergence of Fuller's mystical and military interests.