Complex sentence: After trimming began, 15 to 16 KPA soldiers appeared commanded by Senior Lt. Pak Chul, whom the UNC soldiers had nicknamed "Lt. Bulldog" for his 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 are claimed to have called him "Lt. Bulldog" for his history of confrontations.