Complex sentence: The Union defenders were secluded in the underbrush along the slope of a rugged hill, but as the Confederate attack came forward, the Native Americans began to fall back, taking cover for a while and then moving back.
Split into simpler sentences: The Union defenders were secluded in the underbrush along the slope of a rugged hill. As the Confederate attack progressed, the Native Americans began to fall back, taking cover for a while and then moving back.