Legio XXI Rapax was a legion of the Imperial Roman army. The symbol of the legion is thought to have been a capricorn.

The Battle of the Teutoburg Forest, described as the Varian Disaster by Roman historians, was a major battle between Germanic tribes and the Roman Empire that took place somewhere near modern Kalkriese from September 8–11, 9 AD, when an alliance of Germanic peoples ambushed three Roman legions led by Publius Quinctilius Varus and their auxiliaries. The alliance was led by Arminius, a Germanic officer of Varus's auxilia. Arminius had acquired Roman citizenship and had received a Roman military education, which enabled him to deceive the Roman commander methodically and anticipate the Roman army's tactical responses.