Complex sentence: Chun formed Hanahoe as a secret military club shortly after his promotion to general officer, made up mostly of graduates from his 11th class of the Korea Military Academy and other friends and supporters.
Split into simpler sentences: Chun formed Hanahoe as a secret military club shortly after his promotion to general officer. It was predominantly composed of his fellow graduates from the 11th class of the Korea Military Academy, as well as other friends and supporters.