Battle of the Teutoburg Forest
The Battle of the Teutoburg Forest was the worst defeat to befall the Roman Empire during the reign of Augustus. It occurred while Rome was enjoying the Pax Romana in 9 A.D. and moving further into Germany for colonization.
Varus led a Roman force of perhaps 20,000 men along with 10,000 non-combatant family members who went with them. They marched north of the Visurgis river into the wooded mountains Teutoberger region with his German ally Arminius. During a heavy storm, Arminius and his men suddenly turned on a Roman detachment and annihilated it as they left. Varus learned his camp had been invested and tried to cut his way through the forest to create a new camp on the North Sea Coast. Slowed by his non-combatants, horrible weather, and lack of adequate forest trails, the Romans were constantly harassed by the Germanic tribes who were quite familiar with the terrain and territory. After days of this harassment, the Germans finally broke through and annihilated almost the entire Roman army, including the non-combatants.
From 9 A.D. to 13 A.D., Tiberius and then Germanicus made punitive expeditions against the Germans. Nevertheless, Augustus, near the end of his life, had already made the decision to put the boundaries of the Empire along the Rhine and Danube rivers and abandon the idea of the colonization of Germany. This decision was followed by his successors. Due to this decision and the change in Roman plans, the Battle of the Teutoburg Forest is considered to be one of the decisive battles in history.
Encyclopedia of Military History, Dupuy & Dupuy, 1979