Complex sentence: In January 1944, he joined the 3rd Marine Division at Guadalcanal, attached to the 1st Battalion, 21st Marines, first to Company C and then to Headquarters Company.
Split into simpler sentences: In January 1944, he joined the 3rd Marine Division at Guadalcanal. He was attached to the 1st Battalion, 21st Marines, first to Company C and then to Headquarters Company.