The Battle of the Teutoburg Forest, described as the Varian Disaster (Clades Variana) by Roman historians, took place at modern Kalkriese in AD 9, when an alliance of Germanic peoples ambushed Roman legions and their auxiliaries, led by Publius Quinctilius Varus. 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.

Teutoburg Forest is commonly seen as one of the most important defeats in Roman history, bringing the triumphant period of expansion under Augustus to an abrupt end. The outcome of this battle dissuaded the Romans from their ambition of conquering Germania, and is thus considered one of the most important events in European history. The provinces of Germania Superior and Germania Inferior, sometimes collectively referred to as Roman Germania, were subsequently established in northeast Roman Gaul, while territories beyond the Rhine remained independent of Roman control. Retaliatory campaigns were commanded by Tiberius and Germanicus and would enjoy success, but the Rhine would eventually become the border between the Roman Empire and the rest of Germania. The Roman Empire would launch no other major incursion into Germania until Marcus Aurelius (r. 161180) during the Marcomannic Wars.Some of the descendants of the vassal kingdoms, like the Suebi (by suzerainty), that Augustus tried to create in Germania to expand the romanitas and the Empire in a peaceful way would be the ones that invaded the Empire in the fourth and fifth centuries.

Arminius (German: Hermann, ; 18/17 BC – 21 AD) was a Roman officer and later 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 9 AD, in which three Roman legions under the command of general Publius Quinctilius Varus were destroyed. His victory at Teutoburg Forest would precipitate the Roman Empire's permanent strategic withdrawal from Germania Magna. Modern historians have regarded Arminius' victory 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 world history.Born a prince of the Cherusci tribe, Arminius was part of the Roman friendly faction of the tribe. He learned Latin and served in the Roman military, which gained him Roman citizenship and the rank of equite. After serving with distinction in the Great Illyrian Revolt, he was sent to Germania to aid the local governor Publius Quinctilius Varus in completing the Roman conquest of the Germanic tribes. While in this capacity, Arminius secretly plotted a Germanic revolt against Roman rule, which culminated in the ambush and destruction of three Roman legions in the Teutoburg Forest.

In the aftermath of the battle, Arminius fought retaliatory invasions by the Roman general Germanicus in the battles of Pontes Longi, Idistaviso, and the Angrivarian Wall, and deposed a rival, the Marcomanni king Maroboduus. Germanic nobles, afraid of Arminius' growing power, assassinated him in 21 AD. He was remembered in Germanic legends for generations afterwards. The Roman historian Tacitus designated Arminius as the liberator of the Germanic tribes and commended him for having fought the Roman Empire to a standstill at the peak of its power.During the unification of Germany in the 19th century, Arminius was hailed by German nationalists as a symbol of German unity and freedom. Following World War II, however, Arminius was omitted from West German textbooks due to his association with militaristic nationalism; the 2,000th anniversary of his victory at the Teutoburg Forest was only lightly commemorated in Germany.