At the Battle of Teutoburg Forest (aka Battle of Varus), c. 9 CE, a combined force of Germans annihilated a Roman army consisting of three legions including three squadrons of cavalry and six cohorts of auxiliary troops. As some soldiers must have been left behind to defend summer camps, the army probably held 10,000 to 15,000 men or roughly 8 to10 % of the total Roman army. The sources do not mention the size of the German army at the famous battle. Based on the size of the catchment area, and the fact that Arminius, leader of the German forces, was not able to recruit all chieftains, it is likely that the Germans were heavily outnumbered, perhaps 1:2.
This is a companion discussion topic for the original content at https://www.worldhistory.org/article/1010/battle-of-teutoburg-forest/.