Complex sentence: As the Nervii pursued them uphill Caesar reminded the legionaires that they were crowded too closely together and thus not fighting effectively; he held them fast and ss he tells it, it was only his arrival with the XII and VII that saved the day.
Split into simpler sentences: As the Nervii pursued them uphill Caesar reminded the legionaires they were crowded too closely together and thus not fighting effectively. As he tells it, it was only his arrival with the XII that saved the day.