Complex sentence: Napoleon took great care of his Guard, particularly the Old Guard, and the Old Guard Grenadiers were known to complain in the presence of the Emperor, giving them the nickname "Les Grognards", the Grumblers.
Split into simpler sentences: Napoleon took great care of his Guard, particularly the Old Guard. The Grenadiers of Old Guard were known to complain in the presence of the Emperor, giving them the nickname "Les Grognards", the Grumblers.