why did the romans use an eagle as a symbol
Aquila (Roman) The eagle was extremely important to the Roman military, beyond merely being a symbol of a legion. A lost standard was considered an extremely grave occurrence, and the Roman military often went to great lengths to both protect a standard and to recover it if lost; for example, see the aftermath of the Battle of the Teutoburg Forest, where the Romans spent decades attempting to recover the lost standards of three legions.
Roman mythology The Aeneid and Livy's early history are the best extant sources for Rome's founding myths. Material from Greek heroic legend was grafted onto this native stock at an early date. The Trojan prince Aeneas was cast as husband of Lavinia, daughter of King Latinus, patronymical ancestor of the Latini, and therefore through a convoluted revisionist genealogy as forebear of Romulus and Remus. By extension, the Trojans were adopted as the mythical ancestors of the Roman people.[8]