Complex sentence: An attempted storm failed and he led a disastrous winter retreat, losing between 1000 and 5000 of his 8000 men and carrying 1000 sick and wounded, most of whom never recovered.
Split into simpler sentences: An attempted storm failed and he led a disastrous winter retreat. He lost between 1000 and 5000 of his 8000 men and they had to carry back 1000 sick and wounded, most of whom never recovered.