This legion was probably created to deal with Sextus Pompeius, the last opponent of the second triumvirate, garrisoned in Sicily and threatening Rome's grain supply. It was probably one of the eight legions Augustus promised Mark Antony for his campaign against the Parthians, but never delivered.
In September 9 AD, Arminius, leader of the Cheruscan allies, reported a rebellion in the Rhine area. Without suspecting the information received, Varus took his three legions, the XVIIIth along with Legio XVII and Legio XIX, and headed west. On September 9, near modern Osnabrück, the Cheruscii led by Arminius ambushed the governor's army. All three legions were destroyed in what is known as the battle of Teutonburg Forest and their eagle standards lost.
Later, Germanicus sent an expedition to the place and recovered the eagles for Rome.
Vindonissa was occupied from 46 to 69 by the Legio and two auxiliary cohorts, first the III Hispanorum and VI Raetorum, and later the VII Raetorum equitata and the XXVI voluntariorum civium Romanorum.
Legio XXI Rapax was part of 40,000 Roman soldiers loyal to Vitellius that marched on Rome and were victorious.
Legio XXI Rapax's legionary symbol is the Capricorn.
LEGIO VII HISPANA later GEMINA "Twin" Raised in Spain (Hispana) by Galba in 68, the Legion was nicknamed "Galbiana".
LEGIO XVI GALLICA Raised by Octavian in 41-40 BC; the Legion, from 30 BC onwards, was stationed on the Rhine frontier.
The remainder of the Legion surrendered to the rebel Iulius Civilis in 69 and was subsequently disbanded by Emperor Vespasian and reconstituted as Legion XVI Flavia Firma "Flavian Steadfast".