The Roman legion, the largest military unit of the Roman army, was composed of Roman citizens serving as legionaries. During the Roman Republic the manipular legion comprised 4,200 infantry and 300 cavalry. After the Marian reforms in 107 BC the legions were formed of 5200 men and were restructured around 10 cohorts, the first cohort being double strength. This structure persisted throughout the Principate and Middle Empire, before further changes in the fourth century resulted in new formations of around 1000 men.

Arminius was a chieftain of the Germanic Cherusci tribe who is best known for commanding an alliance of Germanic tribes at the Battle of the Teutoburg Forest in AD 9, in which three Roman legions under the command of general and governor Publius Quinctilius Varus were destroyed. His victory at Teutoburg Forest precipitated the Roman Empire's permanent strategic withdrawal from Germania Magna, and modern historians regard it as one of Rome's greatest defeats. As it prevented the Romanization of Germanic peoples east of the Rhine, it has also been considered one of the most decisive battles in history and a turning point in human history.