The Roman eagle has multiple, layered roles as a symbol in The Eagle of the Ninth. Eagles were a prevalent image throughout Rome but were most appreciably associated with the Roman Army. Each Roman legion, or division of 4,000 soldiers, carried its own sacred eagle, a carved figure of the bird of prey mounted on a standard. The loss or destruction of this eagle was catastrophic for a legion—not only an affront to the Empire, a loss of dignity, and a severe detriment to the legion’s reputation, but also a failure imbued with religious and superstitious implications. Each legion regarded its eagle with the same significance and reverence, and thus this symbolism is collectively understood and appreciated by those familiar enough with Roman customs in The Eagle of the Ninth, but the individual eagle of Marcus’s father’s legion is itself a symbol for the bewildering disappearance of the entire legion.

At the beginning of the novel, Marcus reasons with himself that perhaps, wherever his father and the Ninth Legion might be, they may yet have the eagle in their possession. If this were true, it would mean that, having never been separated from their eagle, he may yet be justified in defending his father and the legion as a whole.