Complex sentence: Meanwhile, the 7th Marines opened the road between Hagaru-ri and Koto-ri by capturing the high ground surrounding the road, but as soon as the Marines pulled out, the 77th Division returned to the peaks and attacked the column.
Split into simpler sentences: Meanwhile, the 7th Marines opened the road between Hagaru-ri and Koto-ri by capturing the high ground surrounding the road. But as soon as the Marines pulled out, the 77th Division returned to the peaks and attacked the column.