One of Napoléon's main symbols was the eagle, which was also closely associated with the Roman Empire. The Roman soldiers carried an eagle on a staff into battle, and being the aquilifer (literally "eagle-bearer") was a position of honor. If the eagle was lost during battle, this was considered a colossal failure and blow to morale. Napoléon resurrected all of this. He gave his regiments eagle-topped standards that looked like this starting in December 1804. He made his soldiers swear oaths to protect the eagle at all costs, and chewed out one battalion for losing it at Austerlitz. Source

Here's one of David's paintings showing the aforementioned oath to protect the eagles. Note the Emperor's posture, which recalls depictions of Roman emperors, as well as the huge eagles adorning the columns.