Complex sentence: After trimming began, 15 to 16 KPA soldiers appeared, commanded by Senior Lt. Pak Chul, whom the UNC soldiers had previously nicknamed'Lt. Bulldog' due to a history of confrontations.
Split into simpler sentences: After trimming began, 15 to 16 KPA soldiers commanded by Senior Lt. Pak Chul appeared. The UNC soldiers had previously nicknamed him'Lt. Bulldog', for a history of confrontations.